tag:blogger.com,1999:blog-90167261456367019982023-11-16T09:41:11.305-08:00Universe at a glanceThe blog covers the subjects of Astronomy, Physics, Astrophysics,Instrumentation and Methods for Astrophysics, Cosmology, High-Energy Astrophysics, Observation methods, Asrobiology, and recent science news.Olga V. Vovkhttp://www.blogger.com/profile/10788084870025950079noreply@blogger.comBlogger9125tag:blogger.com,1999:blog-9016726145636701998.post-42060591213589898062011-10-05T06:34:00.000-07:002011-10-06T05:23:44.314-07:00Nobel Prize in physics and ESA Cosmic Vision<div dir="ltr" style="text-align: left;" trbidi="on"><br />
<div class="MsoNormal">Two things happened on October 4, 2011 – </div><ol start="1" style="margin-top: 0in;" type="1"><li class="MsoNormal">the Nobel committee announced that its physics award goes to Saul Perlmutter and Adam Riess of the US and Brian Schmidt from Australia for the research that identified the "accelerating expansion of the Universe", see <a href="http://www.bbc.co.uk/news/science-environment-15165371">http://www.bbc.co.uk/news/science-environment-15165371</a></li>
<li class="MsoNormal">ESA Cosmic Vision panel approved two middle size missions as a part of its Cosmic Vision 2015-2025 plan. These are Euclid and Solar Orbiter. </li>
</ol><div class="MsoNormal">Saul Perlmutter, Adam Riess and Brian Schmidt studied Type Ia supernovae and found that the most distant of them are moving back quicker that those that are close. This observation, in turn led, to the theory that the Universe is expanding and that some mysterious energy – the “dark energy” must be behind the expansion. However, currently, we have no idea what it might be.</div><div class="MsoNormal">ESA's Euclid is a mission that will investigate this "dark energy", which is believed to be responsible for driving the Universe apart. As is states on the ESA web site “the mission will map out the large-scale structure of the Universe with unprecedented accuracy. The observations will stretch across 10 billion light years into the Universe, revealing the history of its expansion and the growth of structure during the last three-quarters of its history”, see <a href="http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=49385"><span style="color: black; text-decoration: none;">http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=49385</span></a>. Euclid supposes to launch in 2017.</div><div class="MsoNormal">However, the methods the Euclid will use to investigate dark energy (I am dropping “” for dark energy here) are different than those that Noble trio and their colleagues used.</div><div class="MsoNormal">Euclid is designed to study the influence of dark energy by probing Weak gravitational Lensing (WL) and Baryonic Acoustic Oscillations (BAO).<br />
<br />
</div><div class="MsoNormal">- WL allows registering very faint distortions in the way galaxies appear as on the sky, which, in tur,n allows detecting mass inhomogeneities along the line-of-sight.</div><div class="MsoNormal">- BAO are wiggle patterns in the three dimensional distribution of clusters of galaxies. By measuring them, we can determine the redshifts of galaxies with accuracy better than 0.1%. This method can be used as a standard ruler to measure dark energy and the expansion in the Universe.<br />
For more information, refer to <a href="http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=42267"><span style="color: black; text-decoration: none;">http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=42267</span></a></div><div class="MsoNormal"><br />
In terms of instruments WL requires extremely high image quality and BAO requires fine spectroscopy. Both techniques employ fine infrared detectors which ESA currently buys from US.</div><div class="MsoNormal"><br />
The mission that has the concept of using Type Ia supernovae to probe dark energy is the future NASAs Wide-Field Infrared Survey Telescope or WFIRST. </div><div class="MsoNormal"><br />
The Astro2010 Decadal panel identified WFIRST (<a href="http://wfirst.gsfc.nasa.gov/">http://wfirst.gsfc.nasa.gov</a>/) as the top priority mission for the upcoming decade. However, NASA's James Webb Space Telescope is scheduled to launch in 2018 and this large mission is keeping WFIRST from being implemented until perhaps the 2020s.</div><div class="MsoNormal"><br />
As proposed, WFIRST will measure the properties of more than a thousand supernovae which can be used to directly calculate the luminosity distance (DL). On the other hand, certain spectral features in the supernova light can be used to identify z (redshift) and provide the distance-redshift relation D(z), which is a primary observable of the effect of dark energy. For more information, see <a href="http://wfirst.gsfc.nasa.gov/science/de/"><span style="color: black; text-decoration: none;">http://wfirst.gsfc.nasa.gov/science/de/</span></a>.</div><div class="MsoNormal"><br />
But WFIRST is delayed to 2020s the soonest.</div><div class="MsoNormal"><br />
I wonder if S. Perlmutter's and colleagues' Nobel price can help NASA find the money to build WFIRST sooner?</div><div class="MsoNormal"><br />
As alternative, NASA can ask about participation in Euclid. I know that in the past ESA offered NASA a 20% partnership in the mission. NASA, for example, could provide the infrared detectors which are not available in Europe (yet). I don’t think NASA can get more participation, especially after issues with IXO (<a href="http://sci.esa.int/science-e/www/area/index.cfm?fareaid=103">http://sci.esa.int/science-e/www/area/index.cfm?fareaid=103</a>). But it would be great to have at least 20%, although I wonder if it would be possible.</div><div class="MsoNormal"><br />
<b>References:</b></div><span style="float: left; padding: 5px;"><a href="http://www.researchblogging.org"><img alt="ResearchBlogging.org" src="http://www.researchblogging.org/public/citation_icons/rb2_tiny.png" style="border:0;"/></a></span><br />
<div class="MsoNormal"><span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.jtitle=Astrophysical+Journal&rft_id=info%3A%2F&rfr_id=info%3Asid%2Fresearchblogging.org&rft.atitle=Measurements+of+Omega+and+Lambda+from+42+High-Redshift+Supernovae&rft.issn=&rft.date=1998&rft.volume=&rft.issue=&rft.spage=&rft.epage=&rft.artnum=http%3A%2F%2Farxiv.org%2Fabs%2Fastro-ph%2F9812133v1&rft.au=S.+Perlmutter%2C+G.+Aldering%2C+G.+Goldhaber%2C+R.A.+Knop%2C+P.+Nugent%2C+P.G.+Castro%2C+S.+Deustua%2C+S.+Fabbro%2C+A.+Goobar%2C+D.E.+Groom%2C+I.+M.+Hook%2C+A.G.+Kim%2C+M.Y.+Kim%2C+J.C.+Lee%2C+N.J.+Nunes%2C+R.+Pain%2C+C.R.+Pennypacker%2C+R.+Quimby%2C+C.+Lidman%2C+R.S.+Ellis%2C+M.+Irwin%2C+R.G.+Mc&rfe_dat=bpr3.included=1;bpr3.tags=Astronomy%2Cphysics%2C+astrophysics%2C+Instrumentation+and+Methods+for+Astrophysics">S. Perlmutter, G. Aldering, G. Goldhaber, R.A. Knop, P. Nugent, P.G. Castro, S. Deustua, S. Fabbro, A. Goobar, D.E. Groom, I. M. Hook, A.G. Kim, M.Y. Kim, J.C. Lee, N.J. Nunes, R. Pain, C.R. Pennypacker, R. Quimby, C. Lidman, R.S. Ellis, M. Irwin, R.G. Mc (1998). Measurements of Omega and Lambda from 42 High-Redshift Supernovae <span style="font-style: italic;">Astrophysical Journal</span></span></div><div class="MsoNormal">BBC news - http://www.bbc.co.uk/news/science-environment-15180497</div></div>Olga V. Vovkhttp://www.blogger.com/profile/10788084870025950079noreply@blogger.com5tag:blogger.com,1999:blog-9016726145636701998.post-8958224716525323872011-05-27T21:11:00.000-07:002011-06-01T05:42:33.901-07:00Observing Sgr A*<div dir="ltr" style="text-align: left;" trbidi="on"><div dir="ltr" style="text-align: left;" trbidi="on"><br />
<div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> Our image of Sgr A*</span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> </span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">is constrained by what we see and what we do not see (the later is even more important).<o:p></o:p></span></span></div><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">In 1974, two American radio astronomers Bruce Balick and Robert Brown discovered a compact and variable radio source that looked like a faint quasar</span></span><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> in the center of Milky Way</span><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">. Because it appeared to be inside a large, extended radio source already known as Sagittarius A, they named it Sagittarius A* (or </span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Sgr A*)</span></span><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">.</span></div><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> Here I should take a short trip to the basics of radio astronomy.<o:p></o:p></span></span></div><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> First observations in radio frequencies were done by Karl Jansky in 1930. This opens a new field in astronomy and allowed observing stars and galaxies in new range of wavelengths, unveiling new features. Subsequent observations have also identified new classes of objects, such as radio galaxies, quasars, pulsars, and masers.</span><a href="http://www.blogger.com/post-create.g?blogID=9016726145636701998#_ftn1" name="_ftnref" title=""><span class="MsoFootnoteReference"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">[0]</span></span></a><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><o:p></o:p></span></span></div><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> Ground based radio-observations are limited to a range of wavelengths that can go through the Earth atmosphere - 1mm – 10m (or 30 – 300 GHz). At longer wavelengths (lower frequencies), transmission is limited by the ionosphere, which reflects waves with frequencies less than 30GHz, while at higher frequencies (at millimeter range), radio signals are prone to atmospheric absorption.<o:p></o:p></span></span></div><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> In order to observe faint sources with good angular resolution, radio telescopes need to be i) extremely large and ii) be placed in very high and dry sites, e.g. ALMA, VLA, etc.<o:p></o:p></span></span></div><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Angular resolution is a term used to describe the ability of any imaging device to distinguish small details of an object.<o:p></o:p></span></span><br />
<span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br />
</span></span></div><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> <span class="Apple-style-span" style="font-size: x-small;"><b>Imagine two point sources to observe. They will be regarded as resolved when the distance between the principal diffraction maximum of the first source and the first minimum of the second is bigger than zero. If one considers diffraction through a circular aperture, this translates into:<o:p></o:p></b></span></span></span></div><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;"><b>sin </b></span></span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;"><b>q</b></span></span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;"><b> = 1.220 </b></span></span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;"><b>l</b></span></span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;"><b>/D,<o:p></o:p></b></span></span></span></div><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;"><b>Where <o:p></o:p></b></span></span></span></div><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;"><b>θ is the angular resolution in radians,<o:p></o:p></b></span></span></span></div><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;"><b>λ is a wavelength,<o:p></o:p></b></span></span></span></div><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;"><b>and D is the diameter of the aperture.<o:p></o:p></b></span></span></span></div><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;"><b> This means that in order to improve angular resolution one needs either smaller wavelengths (in a millimeter range) or bigger mirrors/dishes or both.</b></span><o:p></o:p></span></span><br />
<span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;"><b><br />
</b></span></span></span></div><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> In order for a telescope to develop a clear image of a distant object, the diameter of the collecting area must be much bigger than the wavelength of its collecting radiation. For an optical telescope, the wavelengths of visible light are in the range of 380 to 780 nm, which makes telescope lenses to be meters across. To provide the same angular resolution using a radio telescope, a single dish antenna would have to be kilometers across.</span><o:p></o:p></span></div><div class="MsoNormal"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> </span></div><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"></span><br />
<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"></span><br />
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<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><div class="MsoNormal"><span style="color: windowtext;"> With inventing the radio-interferometry technique, high-resolution images become possible via employing multiple small antennas, which are connected together to simulate the collective power of one large antenna.<a href="http://www.blogger.com/post-create.g?blogID=9016726145636701998#_ftn1" name="_ftnref" title=""><span class="MsoFootnoteReference">[1]</span></a> This creates a combined telescope which size is actually the distance between the farthest antennas in the array (a baseline).<a href="http://www.blogger.com/post-create.g?blogID=9016726145636701998#_ftn2" name="_ftnref" title=""><span class="MsoFootnoteReference">[2]</span></a> With this method modern radio interferometers, such as VLBI<a href="http://www.blogger.com/post-create.g?blogID=9016726145636701998#_ftn3" name="_ftnref" title=""><span class="MsoFootnoteReference">[3]</span></a>, can achieve resolution up to 0.001 arcsec.<span class="MsoFootnoteReference"> <a href="http://www.blogger.com/post-create.g?blogID=9016726145636701998#_ftn4" name="_ftnref" title="">[4]</a></span><o:p></o:p></span></div><div class="MsoNormal"><span style="color: windowtext;"> The first radio observations of the center of Milky Way were done at 20 cm wavelength and revealed a broad Rosetta-like structure composed of different elements including a remnant of supernova exposure occurred sometime within past 100 000 years. There are no fine points to be seen in these images (see also Melia, Fig 1.5).<o:p></o:p></span></div><div class="MsoNormal"><span style="color: windowtext;"><br />
</span></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgflq5JzyVltrezlMkbIX-I6DrtREpYnlJ1fp2hLL5cIMZFQCNa8UkaV2jwIZFeOP_tLZhK68YUF-t3rrzcxhwNnpaO6klAEFAiOPTKAe7Z-eJ9Qft6w3s1M2Ex0UA9P1sVi27o6iqq4T_y/s1600/Sgra200.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgflq5JzyVltrezlMkbIX-I6DrtREpYnlJ1fp2hLL5cIMZFQCNa8UkaV2jwIZFeOP_tLZhK68YUF-t3rrzcxhwNnpaO6klAEFAiOPTKAe7Z-eJ9Qft6w3s1M2Ex0UA9P1sVi27o6iqq4T_y/s1600/Sgra200.jpg" /></a></div><div class="MsoNormal"><span style="color: windowtext;"><br />
</span></div><div style="mso-element: footnote-list;"><span class="Apple-style-span" style="font-family: Verdana;"> </span><br />
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<span class="Apple-style-span" style="font-family: Verdana;"><div class="MsoCaption"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;">Figure 1. This is one of the first radio images of Sgr A* taken at 20 cm covers the central ≈ 7 parsecs. On the left side of the image, the diffuse source Sgr A-East is a supernova remnant from a star exploded several hundred years ago, fills most of the left side of this panel. The bright spiral-shaped emission toward the right-center of the panel is called Sgr A-West and comes from plasma spiraling inward to the center. </span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;">Images courtesy of J.-H. Zhao</span></span><o:p></o:p></span></div><div class="MsoCaption"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;"> </span></span></span></div><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><div class="MsoCaption"><span class="Apple-style-span" style="font-size: small;"> They also revealed that Sgr A* was extremely compact – less than our Solar System. However, without information from infrared and X-ray part of spectra its nature remained a mystery.<o:p></o:p></span></div><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: small;"> The VLA images of GC rendered at 6 cm show more detail, including 3 spiral arms 3 light-years long. And the image taken at 2cm shows the central 2 light year region, features spiral pattern of Sagittarius A West and the point-like source of radio emission known as Sagittarius A* (Melia, Fig 1.7, 1.8). </span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: small;"><o:p></o:p></span></span></div><div class="MsoNormal" style="margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none; text-indent: 14.2pt;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: small;">Radio observations at 7 mm and 3.5 mm have detected intrinsic structure in Sgr A*, but the spatial resolution of observations at these wavelengths is limited by interstellar scattering. The apparent size of Sgr A* is dominated by scatter broadening at frequencies up to 50 GHz and the smallest size detected (as reported by Doelemann et al.,</span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: small;"> 2008) </span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: small;">is </span></span><span style="color: windowtext; font-family: 'Apple Symbols';"><span class="Apple-style-span" style="font-size: small;"></span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: small;">0.1 AU </span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: small;">at </span></span><span style="color: windowtext; font-family: 'Apple Symbols';"><span class="Apple-style-span" style="font-size: small;"></span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: small;">1.3</span></span><span style="color: windowtext; font-family: 'Apple Symbols';"><span class="Apple-style-span" style="font-size: small;"></span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: small;">mm.</span></span><span style="color: windowtext; font-family: 'Apple Symbols';"><span class="Apple-style-span" style="font-size: small;"></span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: small;"> This is less than the expected apparent size of the event horizon (as observed from 8 kpc), suggesting that most of SgrA* emission may not be centered on the black hole, but arises in the surrounding accretion flow.</span><o:p></o:p></span></div><div class="MsoNormal" style="margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none; text-indent: 14.2pt;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: small;"><br />
</span></span></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjpcuSPRb1cbNpVnd1mBQoGeWQ36CQvcRrzSlexr18LJNX8VvS_zYJgKpczuGO2glpYFSlU9cAhFk7ZnBkQAMBAaA3Bx8cbdezpRIzWRS6r7QvWqsJKislOwVH7hw6Xg2ISrvAFkeFyOGNW/s1600/Sgra36mm.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjpcuSPRb1cbNpVnd1mBQoGeWQ36CQvcRrzSlexr18LJNX8VvS_zYJgKpczuGO2glpYFSlU9cAhFk7ZnBkQAMBAaA3Bx8cbdezpRIzWRS6r7QvWqsJKislOwVH7hw6Xg2ISrvAFkeFyOGNW/s320/Sgra36mm.jpg" width="320" /></a></div><div class="MsoNormal" style="margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none; text-indent: 14.2pt;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: small;"><span class="Apple-style-span" style="font-family: Verdana;"> </span></span></span></div><span class="Apple-style-span" style="font-family: Verdana;"><div class="MsoCaption"><span class="Apple-style-span" style="font-size: x-small;">Figure 2. Radio continuum emission at 3.6 cm from the inner few parsecs of our Galaxy. The bright point source in the center is Sagittarius A. The mini-spiral of emission around the point source is from ionized gas that is in systematic motion about Sgr A*. </span><span style="color: windowtext; font-family: Helvetica;"><span class="Apple-style-span" style="font-size: x-small;">Image courtesy of NRAO/AUI</span><o:p></o:p></span></div><div class="MsoCaption"><span style="color: windowtext; font-family: Helvetica;"><span class="Apple-style-span" style="font-size: x-small;"><br />
</span></span></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhaPPowux4EO_pzEMtVVeVZ5ddN-AUMRDXqg9dTUcTe5qYO3ZDjShB3DUAt5HJCiE3acoDXs6RpcMMx7rLxpQBYTfVfVDYcSfKaMYVPtvTg85_pngLEFGgjO77bI9qXA-mkChv3dFMl6Lpk/s1600/sgraVLA13mm.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhaPPowux4EO_pzEMtVVeVZ5ddN-AUMRDXqg9dTUcTe5qYO3ZDjShB3DUAt5HJCiE3acoDXs6RpcMMx7rLxpQBYTfVfVDYcSfKaMYVPtvTg85_pngLEFGgjO77bI9qXA-mkChv3dFMl6Lpk/s320/sgraVLA13mm.jpg" width="262" /></a></div><div class="MsoCaption"><span style="color: windowtext; font-family: Helvetica;"><span class="Apple-style-span" style="font-size: x-small;"><br />
</span></span></div><div align="center" class="MsoNormal" style="text-align: center;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;"><span class="Apple-style-span" style="font-family: Verdana;"><span class="Apple-style-span" style="font-size: small;"> </span></span></span></span></div><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-family: Verdana;"><div class="MsoCaption" style="text-align: left;"><a href="http://www.blogger.com/post-edit.g?blogID=9016726145636701998&postID=895822471652532387" name="_Ref168094941"><span class="Apple-style-span" style="font-size: x-small;">Figure </span></a><span class="Apple-style-span" style="font-size: x-small;">3. The nucleus of Milky Way observed with the VLA at 1.3 cm and imaged with an angular resolution of 0.1 arcsec (Zhao, J.-H., & Goss, W. M. 1998, ApJ, 499, L163). Sgr A*, the bright unresolved radio source in the middle of this image</span></div><div class="MsoCaption" style="text-align: left;"><span class="Apple-style-span" style="font-size: x-small;"> </span></div><div class="MsoNormal" style="margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none; text-indent: 14.2pt;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: small;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">The most radiation shown in radio-images is produced by the diffuse hot gas between the stars. In order to see stars themselves, we need to move to optical wavelengths, which cannot actually penetrate the thick gas around GC. The option is to use near IR diapason (Melia, Fig 1.9 -optical, 1.12 –near IR). Sgr A* itself emits very low in near IR, but we can see emission from nearby stars. <o:p></o:p></span></span></span></div><div class="MsoNormal" style="margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none; text-indent: 14.2pt;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: small;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">To transfer Sgr A*’s radio position to infrared images with reasonable accuracy, the multiple images of nine red giant star, which are bright in both the infrared and radio wavelengths, were taken and positions determined. </span></span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: small;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Figure 4</span></span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: small;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> shows the combined IR and radio image of the central 10” with Sgr A* circled. This allowed to determine coordinates of Sgr A* as accurately as ±0.01 arcseconds or ±80 AU.<o:p></o:p></span></span></span></div><div class="MsoNormal" style="margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none; text-indent: 14.2pt;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: small;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br />
</span></span></span></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjSfiS1XeZIqw0jflqDN94665mQPrBR9paTtiU2VQwuc7hd7F_pm8QgQ0Wlql2bpTbC0ZM5oyDblSYIgE-5PWFD49YB_GWC6aTexJPamSzi6HIm2GB-6b2sOpuMJz1569pNmX05dYvwnq6N/s1600/sgrAimagesgraphs.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="210" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjSfiS1XeZIqw0jflqDN94665mQPrBR9paTtiU2VQwuc7hd7F_pm8QgQ0Wlql2bpTbC0ZM5oyDblSYIgE-5PWFD49YB_GWC6aTexJPamSzi6HIm2GB-6b2sOpuMJz1569pNmX05dYvwnq6N/s320/sgrAimagesgraphs.jpg" width="320" /></a></div><div class="separator" style="clear: both; text-align: center;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: small;"><span class="Apple-style-span" style="font-family: Verdana;"><span class="Apple-style-span" style="font-size: x-small;"> </span></span></span></span></div><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-family: Verdana;"><div class="MsoCaption" style="text-align: left;">Figure 4: Three-color composite image of the central 10”. A radio emission map wavelength 3.6 cm; rendered as red, as well as two SofI images in the Ks- (green) and J-bands (blue). The radio continuum data provided with the Very Large Array by Chris De Pree. Credit: ESO. The spectra demonstrate the wide range of stellar types found in the cluster, ranging from main sequence O stars (the star S2 near Sgr A*, oval in image), to the luminous blue variables (IRS 16 SW, lower right), early WN (middle left and WC (top right) Wolf-Rayet stars, to red supergiants (the brightest star IRS 7 at the top/middle of the image), bright asymptotic giant branch stars (IRS 9, lower left) and red giants (IRS 10 EE, top left). Credit: Nelly Mouawad.</div><span class="Apple-style-span" style="font-size: medium;"><span class="Apple-style-span" style="font-size: 16px;"><br />
</span></span></span></span><br />
<div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: small;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">For more than a decade, groups led by Reinhard Genzel in Germany and Andrea Ghez in the USA led </span></span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: small;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">combined radio and IR observations of stellar motions near GC (see Fig 5), producing estimates of the projected velocities (Eckart & Genzel 1996, 1997; Ghez et al. 1998), projected accelerations (Ghez et al. 2000, 2004; Eckart et al. 2002), and 3-dimensional orbital motions (Schodel et al. 2002, 2003; Ghez et al. 2003, 2005).<o:p></o:p></span></span></span></div><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: small;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br />
</span></span></span></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiAoVH9G2NpQqjvCJ3qDUYdM-mOrGOFLUApOUPSgqAzS_xeTFlfoeRuwxkEwdUNrod6tQC7hJxn0OkH39G7zIs6B1ZRIFreBq9sOUMsxxSmPbVARebk7qAXGsLGtIc6UdTqyIzIC-F7haGj/s1600/starsGC.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="317" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiAoVH9G2NpQqjvCJ3qDUYdM-mOrGOFLUApOUPSgqAzS_xeTFlfoeRuwxkEwdUNrod6tQC7hJxn0OkH39G7zIs6B1ZRIFreBq9sOUMsxxSmPbVARebk7qAXGsLGtIc6UdTqyIzIC-F7haGj/s320/starsGC.jpg" width="320" /></a></div><div class="separator" style="clear: both; text-align: center;"><br />
</div><div class="MsoNormal"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif; font-size: small;"> </span></div><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: x-small;">Figure </span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: x-small;">5</span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: x-small;">.</span></span><span class="Apple-style-span" style="font-size: x-small;"> </span><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: x-small;">Stars within the 0.02 parsecs of the Galactic center orbiting an unseen mass observed for 13 years. Yearly positions of seven stars are color coded. Both curved paths and accelerations (note the non-uniform spacings between yearly points) are evident. Partial and complete elliptical orbital fits for these stars are indicated with lines. Image courtesy A. Ghez.</span><o:p></o:p></span></div>They found that the orbital paths of stars in vicinity of Sgr A* (0.02 pc) are almost perfect ellipses, and most of the unseen mass must be contained within a radius of about 0.0005 pc. This implies a central mass of 4x10^6 Msun with mass density of > 8°x10^15 Msun pc^-3.<br />
<div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: small;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Based on these estimates, the GC is now known to shelter a considerable concentration of dark mass, associated with the radio source Sgr A*. Observations at 7mm done by Bower et al. (2004) put the lower limit to the mass density of Sgr A* of 1.4 × 10^4 Msun per AU^-3. The sub-minute time scale variability in near-IR observed Yusef-Zadeh et al. (2010) put a strong constraint of 1/8 AU on the size of the region from which this variable emission arises.<o:p></o:p></span></span></span></div><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: small;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Each of these observations provided a stronger and stronger case for a SMBH of 3 – 4*10^6 Msun at the centre of the Milky Way and its association with the unusual radio source Sgr A*.<o:p></o:p></span></span></span></div><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: small;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">References:</span></span></span></div><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: small;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br />
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<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> </span></span><span class="Apple-style-span" style="font-size: 13px;">Bower G. et al. 2004, Science Vol. 304 no. 5671 pp. 704-708 DOI: 10.1126/science.1094023</span> <br />
<div class="MsoNormal" style="margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none; text-indent: 0cm;"><span style="color: windowtext; font-size: 10pt;">R. Genzel, A. Eckart, T. Ott & F. Eisenhauer Mon. Not. Roy. Ast. Soc. 291 (1997) 219.<o:p></o:p></span></div><div class="MsoNormal" style="margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none; text-indent: 0cm;"><span style="color: windowtext; font-size: 10pt;">A. Eckart & R. Genzel, Nature 383 (1996) 415.<o:p></o:p></span></div><div class="MsoNormal" style="margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none; text-indent: 0cm;"><span style="color: windowtext; font-size: 10pt;">A. Eckart & R. Genzel, Mon. Not. Roy. Ast. Soc. 284 (1997) 576.<o:p></o:p></span></div><div class="MsoNormal" style="margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none; text-indent: 0cm;"><span style="color: windowtext; font-size: 10pt;">A. M. Ghez, B. L. Klein, M. Morris & E. E. Becklin Ap. J. 509 (1998) 678.<o:p></o:p></span></div><div class="MsoNormal" style="margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none; text-indent: 0cm;"><span style="color: windowtext; font-size: 10pt;">R. Genzel, C. Pichon, A. Eckart, O. E. Gerhard & T. Ott Mon. Not. Roy. Ast. Soc.317 (2000) 348.<o:p></o:p></span></div><div class="MsoNormal" style="margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none; text-indent: 0cm;"><span style="color: windowtext; font-size: 10pt;">A. M. Ghez, M. Morris, E. E. Becklin, A. Tanner & T. Kremenek Nature 407 (2000)<o:p></o:p></span></div><div class="MsoNormal" style="margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none; text-indent: 0cm;"><span style="color: windowtext; font-size: 10pt;">349.<o:p></o:p></span></div><div class="MsoNormal" style="margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none; text-indent: 0cm;"><span style="color: windowtext; font-size: 10pt;">A. Eckart, R. Genzel, T. Ott & R. Sch¨odel, Mon. Not. Roy. Ast. Soc. 331 (2002)<o:p></o:p></span></div><div class="MsoNormal" style="margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none; text-indent: 0cm;"><span style="color: windowtext; font-size: 10pt;">R. Schodel et al., Nature 419 (2002) 694.<o:p></o:p></span></div><div class="MsoNormal" style="margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none; text-indent: 0cm;"><span style="color: windowtext; font-size: 10pt;">A. M. Ghez, et al., Astronomische Nachrichten 324 (2003) 527.<o:p></o:p></span></div><div class="MsoNormal" style="margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none; text-indent: 0cm;"><span style="color: windowtext; font-size: 10pt;">R. Schoodel, T. Ott, R. Genzel, A. Eckart, N. Mouawad & T. Alexander, Ap. J. 596, 2003 1015.<o:p></o:p></span></div><div class="MsoNormal" style="text-indent: 0cm;"><span style="color: windowtext; font-size: 10pt;">A. M. Ghez et al., Ap. J. 620 (2005) 744.<o:p></o:p></span></div><div class="MsoNormal"><span style="color: windowtext; font-size: 10pt;"><a href="http://arxiv.org/find/astro-ph/1/au:+Yusef_Zadeh_F/0/1/0/all/0/1"><span style="color: windowtext; text-decoration: none;">F. Yusef-Zadeh</span></a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bushouse_H/0/1/0/all/0/1"><span style="color: windowtext; text-decoration: none;">H. Bushouse</span></a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dowell_C/0/1/0/all/0/1"><span style="color: windowtext; text-decoration: none;">C.D. Dowell</span></a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wardle_M/0/1/0/all/0/1"><span style="color: windowtext; text-decoration: none;">M. Wardle</span></a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Roberts_D/0/1/0/all/0/1"><span style="color: windowtext; text-decoration: none;">D. 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Belanger</span></a>, 2006, A Multi-Wavelength Study of Sgr A*: The Role of Near-IR Flares in Production of X-ray, Soft $\gamma$-ray and Sub-millimeter Emission, arXiv:astro-ph/0510787v2<o:p></o:p></span></div><div class="MsoNormal"><span style="color: windowtext; font-size: 10pt;">Yusef-Zadeh F., J. Miller-Jones2, D. Roberts3, M. Wardle4, M. Reid5, K. Dodds-Eden6, D. Porquet7 & N. Grosso, Multi-Wavelength Study of Sgr A*: The Short Time Scale Variability, 2010, The Galactic Center: A Window on the Nuclear Environment of Disk Galaxies. ASP Conference Series<o:p></o:p></span></div><div class="MsoNormal"><span style="color: windowtext; font-size: 10pt;">Doeleman, S. S., Weintroub, J., Rogers, A. E. E., Plambeck, R., Freund, R., Tilanus, R. P. J. et al. 2008, Nature, 455, 70<o:p></o:p></span><br />
<span style="color: windowtext; font-size: 10pt;"><span class="Apple-style-span" style="font-family: Times; font-size: small;"><a href="http://www.blogger.com/post-create.g?blogID=9016726145636701998#_ftnref" name="_ftn1" title=""><span class="MsoFootnoteReference"><span class="Apple-style-span" style="font-size: x-small;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">[0]</span></span></span></a><span class="Apple-style-span" style="font-size: x-small;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> To observe objects in the radio spectrum, several techniques are used: 1) an instrument can be pointed at a certain energetic radio source to analyze its emission; or 2) to image a certain region of the sky, multiple overlapping scans are usually made and put together to form a mosaic image. The type of instruments used depends on the strength of the signal and the amount of detail needed.</span></span></span></span></div><div id="ftn" style="mso-element: footnote;"><div class="MsoFootnoteText"><a href="http://www.blogger.com/post-create.g?blogID=9016726145636701998#_ftnref" name="_ftn1" title=""><span class="MsoFootnoteReference"><span class="Apple-style-span" style="font-size: x-small;">[1]</span></span></a><span class="Apple-style-span" style="font-size: x-small;"> Radio interferometry was developed by British radio astronomer Martin Ryle, and two Australian-born radio astronomers Joseph Lade Pawsey and Ruby Payne-Scott in 1946.</span></div></div><div id="ftn"><div class="MsoFootnoteText"><a href="http://www.blogger.com/post-create.g?blogID=9016726145636701998#_ftnref" name="_ftn2" title=""><span class="MsoFootnoteReference"><span class="Apple-style-span" style="font-size: x-small;">[2]</span></span></a><span class="Apple-style-span" style="font-size: x-small;"> </span><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: x-small;">In order to produce a high quality image, a large variety of baselines required. </span></span><span class="Apple-style-span" style="font-size: x-small;">For example, the Very Large Array has 27 telescopes giving 351 independent baselines at once (WikipediaRadioAstronomyWeb). It has to do with the Fourier transformation and a very detailed account of interferometry can be found in the book by Thompson, Moran and Swenson, “Interferometry and Synthesis in Radio Astronomy”, Wiley-Interscience; 2nd edition (April 2001) ISBN: 0471254924.</span></div></div><div id="ftn"><div class="MsoFootnoteText"><a href="http://www.blogger.com/post-create.g?blogID=9016726145636701998#_ftnref" name="_ftn3" title=""><span class="MsoFootnoteReference"><span class="Apple-style-span" style="font-size: x-small;">[3]</span></span></a><span class="Apple-style-span" style="font-size: x-small;"> VLBI consists of widely separated radio telescopes all around the world connected together and tuned to simultaneously observe the same object.</span></div></div><div id="ftn" style="mso-element: footnote;"><div class="MsoFootnoteText"><a href="http://www.blogger.com/post-create.g?blogID=9016726145636701998#_ftnref" name="_ftn4" title=""><span class="MsoFootnoteReference"><span class="Apple-style-span" style="font-size: x-small;">[4]</span></span></a><span class="FootnoteTextChar"><span class="Apple-style-span" style="font-size: x-small;"> The aperture synthesis technique uses tape recorders synchronized with atomic oscillators instead of cables</span></span><span class="Apple-style-span" style="font-size: x-small;">.</span><o:p></o:p></div></div></div></span><br />
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<span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.jtitle=Science&rft_id=info%3Adoi%2F10.1126%2Fscience.1094023&rfr_id=info%3Asid%2Fresearchblogging.org&rft.atitle=Detection+of+the+Intrinsic+Size+of+Sagittarius+A%2A+Through+Closure+Amplitude+Imaging&rft.issn=0036-8075&rft.date=2004&rft.volume=304&rft.issue=5671&rft.spage=704&rft.epage=708&rft.artnum=http%3A%2F%2Fwww.sciencemag.org%2Fcgi%2Fdoi%2F10.1126%2Fscience.1094023&rft.au=Bower%2C+G.&rfe_dat=bpr3.included=1;bpr3.tags=Astronomy%2Cphysics%2C+astrophysics%2C+Instrumentation+and+Methods+for+Astrophysics">Bower, G. (2004). Detection of the Intrinsic Size of Sagittarius A* Through Closure Amplitude Imaging <span style="font-style: italic;">Science, 304</span> (5671), 704-708 DOI: <a href="http://dx.doi.org/10.1126/science.1094023" rev="review">10.1126/science.1094023</a></span><br />
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<span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.jtitle=Nature&rft_id=info%3Adoi%2F10.1038%2Fnature07245&rfr_id=info%3Asid%2Fresearchblogging.org&rft.atitle=Event-horizon-scale+structure+in+the+supermassive+black+hole+candidate+at+the+Galactic+Centre&rft.issn=0028-0836&rft.date=2008&rft.volume=455&rft.issue=7209&rft.spage=78&rft.epage=80&rft.artnum=http%3A%2F%2Fwww.nature.com%2Fdoifinder%2F10.1038%2Fnature07245&rft.au=Doeleman%2C+S.&rft.au=Weintroub%2C+J.&rft.au=Rogers%2C+A.&rft.au=Plambeck%2C+R.&rft.au=Freund%2C+R.&rft.au=Tilanus%2C+R.&rft.au=Friberg%2C+P.&rft.au=Ziurys%2C+L.&rft.au=Moran%2C+J.&rft.au=Corey%2C+B.&rft.au=Young%2C+K.&rft.au=Smythe%2C+D.&rft.au=Titus%2C+M.&rft.au=Marrone%2C+D.&rft.au=Cappallo%2C+R.&rft.au=Bock%2C+D.&rft.au=Bower%2C+G.&rft.au=Chamberlin%2C+R.&rft.au=Davis%2C+G.&rft.au=Krichbaum%2C+T.&rft.au=Lamb%2C+J.&rft.au=Maness%2C+H.&rft.au=Niell%2C+A.&rft.au=Roy%2C+A.&rft.au=Strittmatter%2C+P.&rft.au=Werthimer%2C+D.&rft.au=Whitney%2C+A.&rft.au=Woody%2C+D.&rfe_dat=bpr3.included=1;bpr3.tags=Astronomy%2Cphysics%2C+astrophysics%2C+Instrumentation+and+Methods+for+Astrophysics">Doeleman, S., Weintroub, J., Rogers, A., Plambeck, R., Freund, R., Tilanus, R., Friberg, P., Ziurys, L., Moran, J., Corey, B., Young, K., Smythe, D., Titus, M., Marrone, D., Cappallo, R., Bock, D., Bower, G., Chamberlin, R., Davis, G., Krichbaum, T., Lamb, J., Maness, H., Niell, A., Roy, A., Strittmatter, P., Werthimer, D., Whitney, A., & Woody, D. (2008). Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre <span style="font-style: italic;">Nature, 455</span> (7209), 78-80 DOI: <a href="http://dx.doi.org/10.1038/nature07245" rev="review">10.1038/nature07245</a></span><br />
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<span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.jtitle=Astrophys.J.644%3A198-213%2C2006&rft_id=info%3Aarxiv%2Fastro-ph%2F0510787v2&rfr_id=info%3Asid%2Fresearchblogging.org&rft.atitle=A+Multi-Wavelength+Study+of+Sgr+A%2A%3A+The+Role+of+Near-IR+Flares+in%0D%0A++Production+of+X-ray%2C+Soft+%24%5Cgamma%24-ray+and+Sub-millimeter+Emission&rft.issn=&rft.date=2005&rft.volume=&rft.issue=&rft.spage=&rft.epage=&rft.artnum=&rft.au=F.+Yusef-Zadeh&rft.au=H.+Bushouse&rft.au=C.+D.+Dowell&rft.au=M.+Wardle&rft.au=D.+Roberts&rft.au=C.+Heinke&rft.au=G.+C.+Bower&rft.au=B.+Vila+Vilaro&rft.au=S.+Shapiro&rft.au=A.+Goldwurm&rft.au=G.+Belanger&rfe_dat=bpr3.included=1;bpr3.tags=Astronomy%2Cphysics%2C+astrophysics%2C+Instrumentation+and+Methods+for+Astrophysics">F. Yusef-Zadeh, H. Bushouse, C. D. Dowell, M. Wardle, D. Roberts, C. Heinke, G. C. Bower, B. Vila Vilaro, S. Shapiro, A. Goldwurm, & G. Belanger (2005). A Multi-Wavelength Study of Sgr A*: The Role of Near-IR Flares in<br />
Production of X-ray, Soft gamma$-ray and Sub-millimeter Emission <span style="font-style: italic;">Astrophys.J.644:198-213,2006</span> arXiv: <a href="http://arxiv.org/abs/astro-ph/0510787v2" rev="review">astro-ph/0510787v2</a></span></div>Olga V. Vovkhttp://www.blogger.com/profile/10788084870025950079noreply@blogger.com1tag:blogger.com,1999:blog-9016726145636701998.post-71390958140861982202011-05-05T18:56:00.001-07:002011-05-06T09:07:41.220-07:00Saggitarius A*: distance and mass estimates<div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhNCPrg3gzc5CcFppFPGikFoFFmoJguAuwMmbSxkaTFhm0ofZSN9zbtI4YBAqfvoTRayber6vRy334DxWN6a14C3zAyfNNiMgSI6gel7k3n14stW2SIYqKLBuxrGmrVO71a2Tfp1PDbPdgj/s1600/centerMilkyWay.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhNCPrg3gzc5CcFppFPGikFoFFmoJguAuwMmbSxkaTFhm0ofZSN9zbtI4YBAqfvoTRayber6vRy334DxWN6a14C3zAyfNNiMgSI6gel7k3n14stW2SIYqKLBuxrGmrVO71a2Tfp1PDbPdgj/s320/centerMilkyWay.jpg" width="320" /></a></div><div class="separator" style="clear: both; text-align: center;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;">Center of Milky Way. Credit: </span></span><span class="Apple-style-span" style="-webkit-border-horizontal-spacing: 2px; -webkit-border-vertical-spacing: 2px; line-height: 18px;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;">Stefan Gillessen, Reinhard Genzel, Frank Eisenhauer</span></span></span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br />
</span></span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Knowing distance to Sgr A* (Ro) is very important, because it sets the distance scale for every other distance within Milky Way. The total Galaxy's mass, the Sun's orbital velocity, and luminosities of distant stars rely upon the accurate measurement of Ro.<o:p></o:p></span></span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">A variety of methods have been employed by astronomers to determine Ro. These can be separated into three broad categories: <o:p></o:p></span></span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin-bottom: 0cm; margin-left: 36.0pt; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-list: l3 level1 lfo8; mso-pagination: none; text-autospace: none; text-indent: -18.0pt;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">1.</span><span style="font: normal normal normal 7pt/normal 'Times New Roman';"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: small;"> </span></span></span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">The Shapley method of using “standard candles” measurements of objects thought to be in spherical distribution around the core; <o:p></o:p></span></span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin-bottom: 0cm; margin-left: 36.0pt; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-list: l3 level1 lfo8; mso-pagination: none; text-autospace: none; text-indent: -18.0pt;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">2.</span><span style="font: normal normal normal 7pt/normal 'Times New Roman';"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: small;"> </span></span></span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Computational methods that combine observations with Galactic models to arrive at a distance; <o:p></o:p></span></span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin-bottom: 0cm; margin-left: 36.0pt; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-list: l3 level1 lfo8; mso-pagination: none; text-autospace: none; text-indent: -18.0pt;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">3.</span><span style="font: normal normal normal 7pt/normal 'Times New Roman';"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: small;"> </span></span></span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Direct measurements to objects at the Galactic center. <o:p></o:p></span></span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">I will focus on recent advances in direct measurement techniques.<o:p></o:p></span></span><br />
<span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br />
</span></span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><b>The recent advances in infrared astronomy,</b> </span></span><br />
<span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">such as adaptive optics and imaging spectroscopy, allowed high-resolution imaging of the galactic center with 0.025" angular resolution, which corresponds to a spatial resolution less than 200 AU. With such instrumentation, the orbits of stars in vicinity of Sgr A* can be precisely measured. It is safely to say that Sgr A* provides all of the gravitational attraction in the nearby region, and the motion stars in close proximity is governed solely by this object. Therefore, direct observation of this motion can provide the mass and distance estimates for Sgr A*.<o:p></o:p></span></span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><br />
</div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Ghez et al. (1998) using specially processed near-infrared imagery from the 10 m W. M. Keck telescope identified the position of 90 stars in within 6”x6” region near SgrA*. By following the stars over 2 years, they were able to show that those close to SgrA* have radial velocities as high as 1400 +/- 100 km/s. The high orbital velocities and closeness of the orbits to the central mass allowed Ghez estimate it as 2.6 ± 0.2 x10^6 Msun.<o:p></o:p></span></span><br />
<span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br />
</span></span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">With the mass of the central object estimated, and several years of baseline position and velocity data in hand, Samir and Gould (1999) proposed a direct geometrical method for determination of R0, based on Kepler laws. The similar method has been used for decades to estimate the masses of, and distances to, visual binaries.<o:p></o:p></span></span><br />
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<span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><div class="MsoNormal" style="font-family: Times; margin-bottom: 0cm; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm;"><div style="margin-bottom: 0px; margin-left: 0px; margin-right: 0px; margin-top: 0px;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><b>Keplerian methods</b></span></div></div><div class="MsoNormal" style="font-family: Times; margin-bottom: 0cm; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm;"><div style="margin-bottom: 0px; margin-left: 0px; margin-right: 0px; margin-top: 0px;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">The problem with using Keplerian methods is that we do not directly measure the size of the orbits of distant stars. Instead, we measure the proper motions, which is a measurement of angle rather than length. We also need to know the orbital inclinations.<o:p></o:p></span></span></div></div><div class="MsoNormal" style="font-family: Times; margin-bottom: 0cm; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm;"><div style="margin-bottom: 0px; margin-left: 0px; margin-right: 0px; margin-top: 0px;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Using only the proper motions and orbital periods of the stars near Sgr A*, we could only derive the ratio mass Sgr A*/R0^3.<o:p></o:p></span></span></div></div><div class="MsoNormal" style="font-family: Times; margin-bottom: 0cm; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm;"><div style="margin-bottom: 0px; margin-left: 0px; margin-right: 0px; margin-top: 0px;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">We have, however, one more piece of information: a Doppler shift for each star, as it moves toward or away from us, permits to directly calculate both the mass of Sgr A* and the distance R0 from Earth to Sgr A*.</span></span></div><div style="margin-bottom: 0px; margin-left: 0px; margin-right: 0px; margin-top: 0px;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br />
</span></span></div><div style="margin-bottom: 0px; margin-left: 0px; margin-right: 0px; margin-top: 0px;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><b>Recent results</b></span></span></div></div></span></span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><br />
</div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">The followed continuous monitoring of the star positions proofed that stars do follow elliptical orbits with the focus close to Sgr A*. Also stars acceleration vectors were found directed to a common central gravitational source very close to the position of Sgr A* (Ghez et al. 2005). </span></span><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">The orbital periods of observed stars are ranging from 15 to 94 years. The star with the shortest period (~ 15 years), labeled S2, has already been observed completing its orbit.</span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br />
</span></span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;"><b>Note</b> that t</span></span></span><span class="Apple-style-span" style="line-height: 15px;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;">he </span></span><em style="font-style: normal;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;">Sun's</span></span></em><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;"> orbits around the galaxy center with a speed 220 km/s and its </span></span><em style="font-style: normal;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;">orbital period</span></span></em><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;"> is about 240 million years.</span></span></span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><br />
</div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Schodel et al (2002) used high-resolution near-infrared imaging and spectroscopy to observe the central few light years of our Milky Way and study of the stellar dynamics in the vicinity of the compact radio source SgrA*. From a statistical analysis of the stellar proper motions and Doppler motions they inferred the presence of a compact mass of 2.6- 3.3 10^6 Msun plus the visible stellar cluster of core radius 0.34 pc, located in a region confined within ten light days of SgrA*. <o:p></o:p></span></span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><o:p><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br />
</span></o:p></span></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiyIdOCPlcuUyGPTr8CR_ix8srkh5ex8_A-h1Vp5d_BW9nyYQevLiWxWNYO97700bqN8bJz9UnO17QMZFpysfcw9E9x03B2po-RDpWH-5VubMcZu4Qjoi0O0s-ijMB7muyaP_aSm07tDdrN/s1600/sgrAbig.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiyIdOCPlcuUyGPTr8CR_ix8srkh5ex8_A-h1Vp5d_BW9nyYQevLiWxWNYO97700bqN8bJz9UnO17QMZFpysfcw9E9x03B2po-RDpWH-5VubMcZu4Qjoi0O0s-ijMB7muyaP_aSm07tDdrN/s320/sgrAbig.jpg" width="288" /></a></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><br />
</div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"></div><div style="text-align: center;"><span class="Apple-style-span" style="line-height: 15px;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;">Motion of a star around the galactic center,</span></span></span></div><span style="color: windowtext;"></span><br />
<span style="color: windowtext;"></span><br />
<span style="color: windowtext;"></span><br />
<span style="color: windowtext;"><div style="text-align: center;"><span class="Apple-style-span" style="line-height: 15px;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;">demonstrating that Sagittarius A* is a black hole</span></span></span></div><o:p><div style="text-align: center;"><span class="Apple-style-span" style="line-height: 15px;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;">(adapted from Schödel et al, </span></span></span><span class="Apple-style-span" style="line-height: 15px;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;">Nature</span></span></span><span class="Apple-style-span" style="line-height: 15px;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;">, 17 Oct 2002)</span></span></span></div><div style="text-align: center;"><span class="Apple-style-span" style="line-height: 15px;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;"><br />
</span></span></span></div></o:p></span><br />
<div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Eisenhauer, F. et al. (2005) used near IR imaging spectroscopy with astrometric accuracy of 75 mas to observe the central 30 light-days close to the galactic center. They determined radial velocities for 9 of 10 stars in the central 0.4”, and for 13 of 17 stars out to 0.7”, limiting stars magnitudes to K~16. By combining the new radial velocities with SHARP/NACO astrometry and using a global-fit technique, they derived improved three-dimensional stellar orbits for 6 S stars in the central 0.5” region. This result in the updated estimate for the distance to the Galactic center from the S2 orbit fit as Ro= 7,62 +-0,32 kpc, with a central mass of (3,61+- 0,32)x 10^6 Msun.<o:p></o:p></span></span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br />
</span></span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;">The instrumentation they used is SINFONI - a near-infrared (1.1 - 2.45 µm) integral field spectrograph connected to an adaptive optics module, installed on ESO VLT. The instrument operates with 4 gratings (J, H, K, H+K) providing a spectral resolution around 2000, 3000, 4000 in J, H, K, respectively, and 1500 in H+K. For more information about SINFONI, please refer to the following page: </span></span><a href="http://www.eso.org/sci/facilities/paranal/instruments/sinfoni/overview.html"><span style="text-decoration: none;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;">http://www.eso.org/sci/facilities/paranal/instruments/sinfoni/overview.html</span></span></span></a><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><o:p></o:p></span></span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><br />
</div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">After more than 16 yeas of observations, it was confirmed that stars in vicinity of Sgr A* are executing elliptical orbits, which fit well with a single mass enclosed in a very small volume at the focal position. Two stars have been observed to approach within 100 AU of the focal position, moving at nearly 10^4 km/s responding to an unseen compact mass of 4x10^6 Msun (</span><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Reid 2008, Ghez 2000, Shodel 2003).</span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><b><br />
</b></span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-weight: bold;">How accurate these estimates are?</span></span></span><br />
<span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br />
</span></span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">The error in the proper motion component of the stellar velocities depends primarily on the angular resolution of the detector. Due to advances in technology these errors have come down from 6-10 mas in 1998 to 1-3 mas in 2003 (Eisenhauer et al 2003).<o:p></o:p></span></span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><br />
</div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">The telescopic resolution also limits the accuracy of the stellar positions when they are very close to the central black hole or when two stars are close together in the crowded field because the two sources can blend together (Reid 2008).<o:p></o:p></span></span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><br />
</div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Salim and Gould (1999) published a careful statistical analysis of how the error in R0 should decrease with increasing observational time - after 8 years of observation, the accuracy should be within 2.5% of the actual value, and after 10 years should be within 1%. They also suggested that the accuracy would improve with advances in technology and with larger telescopes, eventually yielding 0.2% error. <o:p></o:p></span></span><br />
<span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br />
</span></span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">This was recently confirmed by Gillessen et al. (2009), who reported the mass estimate for Sgr A* as 4.31 +- 0.06 * 10^6 MSun with statistical error of 1.5%, with the best estimate for R0 = 8.33 +- 0.35 kpc. These estimates were results of 16 years of monitoring stellar orbits around Sgr A* using high-resolution NIR techniques with astrometric accuracy of 300 mas.<o:p></o:p></span></span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><br />
</div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br />
</span></span><br />
<span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><b>Any alternatives to SMBH?</b></span></span><br />
<span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><b><br />
</b></span></span><br />
<span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">According to Shodel et al. (2002), the only possible non-black hole explanation of huge mass of Sgr A* can be a ball of bosons of similar mass, because its radius can be only a few times greater than the Schwarzschild radius of a black hole.<o:p></o:p></span></span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><br />
</div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Another prove for SMBH was analysis of the motion of Sgr A* relative to distance quasars. Sgr A* appears to be almost motionless moving 0.006379” per year (Reid & Brunthaler 2004). This is only possible if Sgr A* contains a huge unseen mass.<o:p></o:p></span></span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><br />
</div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">There is still a possibility that it might be a cluster of dark stars, e.g. a globular cluster. Because a typical GC to could contain 10^6 stars within a radius of 1 parsec. But there is a time constrain - dense star clusters undergo significant interactions, including core-collapse, collisions and evaporation of stars. For the cluster of stars with masses less of 1Msun (note that they should be low luminous starts, even dark ones), to achieve the current conditions close to Sgr A*, the evaporation time should be < 10^6 years, which seems too short. However, a quasi-steady state condition is possible, where stars feed the cluster from the outside at a rate comparable to the evaporation rate (Reid 2008).<o:p></o:p></span></span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><br />
</div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><br />
</div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">References:<o:p></o:p></span></span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"></div><ul><li><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Salim, Samir, and Gould, Andrew. “Sagittarius A</span></span><sup><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: small;">*</span></span></span></sup><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> ‘Visual Binaries’: A Direct Measurement of the Galactocenric Distance.” </span><i><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">The Astrophysical Journal</span></i><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> 523 (1 October 1999): 633—641.</span></span></li>
<li><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Ghez , A. M.; Klein, B. L.; Morris, M.; Becklin, E. E., 1998 ApJ,509:678–686</span></li>
<li><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Eisenhauer, F. et al. 2005, </span><a href="http://adsabs.harvard.edu/abs/2005ApJ...628..246E"><span style="text-decoration: none;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">"SINFONI in the Galactic Center: Young Stars and Infrared Flares in the Central Light-Month"</span></span></a><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">. AJ. 628</span></li>
<li><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Shodel et al., 2002, A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way, Letters to nature</span></li>
<li><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">A. M. Ghez, M. Morris, E. E. Becklin, A. Tanner & T. Kremenek Nature 407 (2000)</span></li>
<li><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">R. Schoodel, T. Ott, R. Genzel, A. Eckart, N. Mouawad & T. Alexander, Ap. J. 596, (2003) 1015.</span></li>
<li><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">M. J. Reid, K. M. Menten, R. Genzel, T. Ott, R. Schodel & Eckart, A. Ap. J. 587, (2003), 208.</span></li>
<li><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Gillessen, S.; Eisenhauer; Trippe; Alexander; Genzel; Martins; Ott (2009). "Monitoring Stellar Orbits Around the Massive Black Hole in the Galactic Center". </span><i><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">The Astrophysical Journal</span></i><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> </span><b><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">692</span></b><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> (2): 1075–1109. </span><a href="http://en.wikipedia.org/wiki/ArXiv"><span style="color: #0842b5; text-decoration: none;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">arXiv</span></span></a><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">:</span><a href="http://arxiv.org/abs/0810.4674"><span style="color: #3464c2; text-decoration: none;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">0810.4674</span></span></a><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">. </span><a href="http://en.wikipedia.org/wiki/Bibcode"><span style="color: #0842b5; text-decoration: none;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Bibcode</span></span></a><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> </span><a href="http://adsabs.harvard.edu/abs/2009ApJ...692.1075G"><span style="color: #3464c2; text-decoration: none;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">2009ApJ...692.1075G</span></span></a><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">. </span><a href="http://en.wikipedia.org/wiki/Digital_object_identifier"><span style="color: #0842b5; text-decoration: none;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">doi</span></span></a><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">:</span><a href="http://dx.doi.org/10.1088%2F0004-637X%2F692%2F2%2F1075"><span style="color: #3464c2; text-decoration: none;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">10.1088/0004-637X/692/2/1075</span></span></a><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">.</span></li>
<li><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">http://einstein.stanford.edu/SPACETIME/spacetime3.html</span></li>
</ul><br />
<br />
<span style="float: left; padding-bottom: 5px; padding-left: 5px; padding-right: 5px; padding-top: 5px;"><a href="http://www.researchblogging.org/"><img alt="ResearchBlogging.org" src="http://www.researchblogging.org/public/citation_icons/rb2_tiny.png" style="border: 0;" /></a></span><br />
<br />
<span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.jtitle=The+Astrophysical+Journal&rft_id=info%3Adoi%2F10.1088%2F0004-637X%2F692%2F2%2F1075&rfr_id=info%3Asid%2Fresearchblogging.org&rft.atitle=MONITORING+STELLAR+ORBITS+AROUND+THE+MASSIVE+BLACK+HOLE+IN+THE+GALACTIC+CENTER&rft.issn=0004-637X&rft.date=2009&rft.volume=692&rft.issue=2&rft.spage=1075&rft.epage=1109&rft.artnum=http%3A%2F%2Fstacks.iop.org%2F0004-637X%2F692%2Fi%3D2%2Fa%3D1075%3Fkey%3Dcrossref.964d8bd98cab29742a0e1e712658e4b1&rft.au=Gillessen%2C+S.&rft.au=Eisenhauer%2C+F.&rft.au=Trippe%2C+S.&rft.au=Alexander%2C+T.&rft.au=Genzel%2C+R.&rft.au=Martins%2C+F.&rft.au=Ott%2C+T.&rfe_dat=bpr3.included=1;bpr3.tags=Astronomy%2Cphysics%2C+astrophysics%2C+Instrumentation+and+Methods+for+Astrophysics">Gillessen, S., Eisenhauer, F., Trippe, S., Alexander, T., Genzel, R., Martins, F., & Ott, T. (2009). MONITORING STELLAR ORBITS AROUND THE MASSIVE BLACK HOLE IN THE GALACTIC CENTER <span style="font-style: italic;">The Astrophysical Journal, 692</span> (2), 1075-1109 DOI: <a href="http://dx.doi.org/10.1088/0004-637X/692/2/1075" rev="review">10.1088/0004-637X/692/2/1075</a></span><br />
<br />
<span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.jtitle=Astrophys.J.+628+%282005%29+246-259&rft_id=info%3Aarxiv%2Fastro-ph%2F0502129v1&rfr_id=info%3Asid%2Fresearchblogging.org&rft.atitle=SINFONI+in+the+Galactic+Center%3A+young+stars+and+IR+flares+in+the+central%0D%0A++light+month&rft.issn=&rft.date=2005&rft.volume=&rft.issue=&rft.spage=&rft.epage=&rft.artnum=&rft.au=F.+Eisenhauer&rft.au=R.+Genzel&rft.au=T.+Alexander&rft.au=R.+Abuter&rft.au=T.+Paumard&rft.au=T.+Ott&rft.au=A.+Gilbert&rft.au=S.+Gillessen&rft.au=M.+Horrobin&rft.au=S.+Trippe&rft.au=H.+Bonnet&rft.au=C.+Dumas&rft.au=N.+Hubin&rft.au=A.+Kaufer&rft.au=M.+Kissler-Patig&rft.au=G.+Monnet&rft.au=S.+Stroebele&rft.au=T.+Szeifert&rft.au=A.+Eckart&rft.au=R.+Schoedel&rft.au=S.+Zucker&rfe_dat=bpr3.included=1;bpr3.tags=Astronomy%2Cphysics%2C+astrophysics%2C+Instrumentation+and+Methods+for+Astrophysics">F. Eisenhauer, R. Genzel, T. Alexander, R. Abuter, T. Paumard, T. Ott, A. Gilbert, S. Gillessen, M. Horrobin, S. Trippe, H. Bonnet, C. Dumas, N. Hubin, A. Kaufer, M. Kissler-Patig, G. Monnet, S. Stroebele, T. Szeifert, A. Eckart, R. Schoedel, & S. Zucker (2005). SINFONI in the Galactic Center: young stars and IR flares in the central<br />
light month <span style="font-style: italic;">Astrophys.J. 628 (2005) 246-259</span> arXiv: <a href="http://arxiv.org/abs/astro-ph/0502129v1" rev="review">astro-ph/0502129v1</a></span>Olga V. Vovkhttp://www.blogger.com/profile/10788084870025950079noreply@blogger.com0tag:blogger.com,1999:blog-9016726145636701998.post-42181663456499114672011-05-03T11:20:00.000-07:002011-05-04T09:28:41.375-07:00Early stars<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> The first generation of stars that formed right after the Big Bang were probably massive luminous stars made of hydrogen and very little helium, they lived short lives (about 30 million years) and after they deaths they provided the Universe with the first heavy (in astronomical terms) elements. </span><br />
<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> It was long thought that these stars lived mostly solitary lives, or formed a very wide binary system. The modern studies suggest that these stars were not only very massive, but also fast rotating.</span><br />
<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> Clark et al (2011), however, provided numerical simulations that show that these stars could be members of tight multiple systems. Their results show that the massive gaseous disks formed around the first rapidly rotating stars were unstable to gravitational fragmentation and could possibly fall into small binary and higher-order systems.</span><br />
<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> All the elements heavier than helium found in later stars were formed by the first stars and recycled in later generations of stars. And however there are no first generation stars left in the nearby Universe to study, we can still reconstruct the chemical composition and masses of first stars using data from the very old stars observable now.</span><br />
<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> Here low mass stars become very handy. Low mass stars (1Msun- 0.5 Msun), live very long lives and contain elements produced by the first generation of stars. They are observable now, gathering in globular clusters, and can be used to provide insights into the lives of the first stars.</span><br />
<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> Chiappini of the Institute for Astrophysics in Potsdam and collaborators used the European Southern Observatory's Very Large Telescope in Chile to study the chemical composition of some of 8 oldest stars from an ancient globular cluster NGC-6522 to figure out what the first stars were like. These stars are old enough to have formed out of the original chemicals produced by the first generation.</span><br />
<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> They found extraordinarily high levels of the heavy elements strontium (Sr) and yttrium (Y) in the surfaces of stars in NGC6522, which suggest that the first stars were both massive and very rapidly rotated to achieve the degree of mixing needed to produce these elements.</span><br />
<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> </span><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Fast stellar rotation makes possible to mix the He-buring core and outer nuclear burning layers, which don’t normally mix in slow rotating stars. The nuclear reactions in the overlapping region leads to an enhanced production of radioactive neon, which emit neutrons that are subsequently captured by Fe and other heavy elements to produce Sr and Y.</span><br />
<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> Chiappini et al. (2011) suggest that, given the abundance of Sr and Y, the first generation of stars could have been rotating as fast as 500 km/s. The typical values for massive stars in the Milky Way are about 100 km/s, and our Sun rotates at 2 km/s.</span><br />
<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> Note that the rapidly rotating stars are more likely to result in gamma-ray bursts, which, in turn, would impact on the ionizing power of the first stars and impact early Universe.</span><br />
<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br />
</span><br />
<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><b>References:</b></span><br />
<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br />
</span><br />
Clark, P., Glover, S., Smith, R., Greif, T., Klessen, R., & Bromm, V. (2011). The Formation and Fragmentation of Disks Around Primordial Protostars <span style="font-style: italic;">Science, 331</span> (6020), 1040-1042 DOI: <a href="http://dx.doi.org/10.1126/science.1198027" rev="review">10.1126/science.1198027</a><br />
<br />
<span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.jtitle=Science&rft_id=info%3Adoi%2F10.1126%2Fscience.1198027&rfr_id=info%3Asid%2Fresearchblogging.org&rft.atitle=The+Formation+and+Fragmentation+of+Disks+Around+Primordial+Protostars&rft.issn=0036-8075&rft.date=2011&rft.volume=331&rft.issue=6020&rft.spage=1040&rft.epage=1042&rft.artnum=http%3A%2F%2Fwww.sciencemag.org%2Fcgi%2Fdoi%2F10.1126%2Fscience.1198027&rft.au=Clark%2C+P.&rft.au=Glover%2C+S.&rft.au=Smith%2C+R.&rft.au=Greif%2C+T.&rft.au=Klessen%2C+R.&rft.au=Bromm%2C+V.&rfe_dat=bpr3.included=1;bpr3.tags=Astronomy%2Cphysics%2C+astrophysics%2C+Instrumentation+and+Methods+for+Astrophysics">Clark, P., Glover, S., Smith, R., Greif, T., Klessen, R., & Bromm, V. (2011). The Formation and Fragmentation of Disks Around Primordial Protostars <span style="font-style: italic;">Science, 331</span> (6020), 1040-1042 DOI: <a href="http://dx.doi.org/10.1126/science.1198027" rev="review">10.1126/science.1198027</a></span><br />
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<span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.jtitle=Nature&rft_id=info%3Adoi%2F10.1038%2Fnature10000&rfr_id=info%3Asid%2Fresearchblogging.org&rft.atitle=Imprints+of+fast-rotating+massive+stars+in+the+Galactic+Bulge&rft.issn=0028-0836&rft.date=2011&rft.volume=472&rft.issue=7344&rft.spage=454&rft.epage=457&rft.artnum=http%3A%2F%2Fwww.nature.com%2Fdoifinder%2F10.1038%2Fnature10000&rft.au=Chiappini%2C+C.&rft.au=Frischknecht%2C+U.&rft.au=Meynet%2C+G.&rft.au=Hirschi%2C+R.&rft.au=Barbuy%2C+B.&rft.au=Pignatari%2C+M.&rft.au=Decressin%2C+T.&rft.au=Maeder%2C+A.&rfe_dat=bpr3.included=1;bpr3.tags=Astronomy%2Cphysics%2C+astrophysics%2C+Instrumentation+and+Methods+for+Astrophysics">Chiappini, C., Frischknecht, U., Meynet, G., Hirschi, R., Barbuy, B., Pignatari, M., Decressin, T., & Maeder, A. (2011). Imprints of fast-rotating massive stars in the Galactic Bulge <span style="font-style: italic;">Nature, 472</span> (7344), 454-457 DOI: <a href="http://dx.doi.org/10.1038/nature10000" rev="review">10.1038/nature10000</a></span><br />
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<span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.jtitle=Nature&rft_id=info%3Adoi%2F10.1038%2Fnature10000&rfr_id=info%3Asid%2Fresearchblogging.org&rft.atitle=Imprints+of+fast-rotating+massive+stars+in+the+Galactic+Bulge&rft.issn=0028-0836&rft.date=2011&rft.volume=472&rft.issue=7344&rft.spage=454&rft.epage=457&rft.artnum=http%3A%2F%2Fwww.nature.com%2Fdoifinder%2F10.1038%2Fnature10000&rft.au=Chiappini%2C+C.&rft.au=Frischknecht%2C+U.&rft.au=Meynet%2C+G.&rft.au=Hirschi%2C+R.&rft.au=Barbuy%2C+B.&rft.au=Pignatari%2C+M.&rft.au=Decressin%2C+T.&rft.au=Maeder%2C+A.&rfe_dat=bpr3.included=1;bpr3.tags=Astronomy%2Cphysics%2C+astrophysics%2C+Instrumentation+and+Methods+for+Astrophysics">Chiappini, C., Frischknecht, U., Meynet, G., Hirschi, R., Barbuy, B., Pignatari, M., Decressin, T., & Maeder, A. (2011). Imprints of fast-rotating massive stars in the Galactic Bulge <span style="font-style: italic;">Nature, 472</span> (7344), 454-457 DOI: <a href="http://dx.doi.org/10.1038/nature10000" rev="review">10.1038/nature10000</a></span><br />
<span style="float: left; padding-bottom: 5px; padding-left: 5px; padding-right: 5px; padding-top: 5px;"><a href="http://www.researchblogging.org/"><img alt="ResearchBlogging.org" src="http://www.researchblogging.org/public/citation_icons/rb2_tiny.png" style="border: 0;" /></a></span>Olga V. Vovkhttp://www.blogger.com/profile/10788084870025950079noreply@blogger.com0tag:blogger.com,1999:blog-9016726145636701998.post-57932536990385548542011-05-01T16:15:00.000-07:002011-05-02T17:35:49.571-07:00Milky Way: a Distance to the Galactic Center - 3<h3><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">First attempts to estimate the distance to the galactic center</span><o:p></o:p></h3><div class="MsoNormal" style="line-height: 19.0pt; margin-bottom: 6.0pt; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext; font-family: Helvetica; font-size: 13pt;">I</span><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">n the late 18th century <b>William Herschel </b>made an attempt to locate the position of Solar System in the Milky Way. His approach was that the center of the galaxy is the place with the highest concentration of stars and by locating such a region he can locate the center of our galaxy. So he looked at all directions and did not find any area of the sky that had a higher concentration of stars than any other area . Based on these observations Hershel concluded that the Earth (or rather the Solar System) must be in the center of our galaxy (HershelWeb). I</span></span><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">n the meantime, he discovered and catalogued over 2400 objects defined by him as nebulae and included in his famous 3 catalogues.</span></div><div class="MsoNormal" style="line-height: 19.0pt; margin-bottom: 6.0pt; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">In 1906, <b>Jacobus Kapteyn</b> began a big project to find the size and the shape of Milky Way, which took him 16 years of intensive observations. He divided the visible sky into 206 zones, and with help of his colleagues from 40 observatories, he surveyed stars in these zones, analyzing their magnitudes, apparent brightness and proper motions. This project was the first coordinated statistical analysis in astronomy.<o:p></o:p></span></span></div><div class="MsoNormal" style="line-height: 19.0pt; margin-bottom: 6.0pt; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">In 1922, the results of this study were finally published: our galaxy was 30,000 light years across, 6000 light years thick, and the Solar System located in its center. Kapteyn’s model of the Milky Way was commonly accepted as accurate for many years.<o:p></o:p></span></span></div><div class="MsoNormal" style="line-height: 19.0pt; margin-bottom: 6.0pt; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">So the Earth as a member of Solar System was again considered a special place – the center of our galaxy. <o:p></o:p></span></span></div><div class="MsoNormal" style="line-height: 19.0pt; margin-bottom: 6.0pt; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><b>Note:</b> while doing his study of proper motions, Kapteyn found that observed stars could be divided into two streams one moving in almost opposite direction to another. These Kapteyn's data were the first evidence of the rotation of our Galaxy (KapteinWeb).<o:p></o:p></span></span></div><div class="MsoNormal" style="line-height: 19.0pt; margin-bottom: 6.0pt; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><b>Harlow Shaple</b>y began his globular cluster survey in 1914, working on the largest telescope this time 60-inch giant at Mt. Wilson Observatory. <o:p></o:p></span></span></div><div class="MsoNormal" style="line-height: 19.0pt; margin-bottom: 6.0pt; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">During his research, Shapley discovered the Cepheid variable stars in a large number of the globular clusters he observed. At that tome the period-luminosity relationship for Cepheids was just reported by Henrietta Swann Leavitt. Using her data Shapley determined the distances to 93 globular clusters (GCs) he observed. He found that the distribution of GCs was centered at about 15 kpc away from the Sun in the direction of the constellation Sagittarius by mapping out the three dimensional distribution of the clusters. This gave Shapley the idea that such massive objects as GCs should be centered at the galactic center. Shapley published his discovery in his "Big Galaxy" theory in 1918 (CudworthWeb).<o:p></o:p></span></span></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh_AecNItbWtOgamHk7RjiWJPg0kQYTUcSD51y0VY9Vx8klJSQKNZITkCrJIg2gBvrNtXKMQ_eKGzfqn9vlIiZwQilVM7DEdyowIL2IG2nAmujAvXMnIkVO9siBcv2bfyrQw_W54xzoXVY3/s1600/GCdistribution.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="241" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh_AecNItbWtOgamHk7RjiWJPg0kQYTUcSD51y0VY9Vx8klJSQKNZITkCrJIg2gBvrNtXKMQ_eKGzfqn9vlIiZwQilVM7DEdyowIL2IG2nAmujAvXMnIkVO9siBcv2bfyrQw_W54xzoXVY3/s320/GCdistribution.jpg" width="320" /></a></div><div class="separator" style="clear: both; text-align: center;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;">A distribution of globular clusters around the center of Milky Way.</span></span></div><div class="MsoNormal" style="line-height: 19.0pt; margin-bottom: 6.0pt; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br />
</span></span></div><div class="MsoNormal" style="line-height: 19.0pt; margin-bottom: 6.0pt; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">However Shapley's conclusions remained controversial at the beginning, they were eventually accepted by majority of astronomers, and his technique is still considered one of the primary means of determining the distance to the center of the Galaxy.<o:p></o:p></span></span></div><div class="MsoNormal" style="line-height: 19.0pt; margin-bottom: 6.0pt; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><b>Jan Oort</b>, while conducted research of the motion of stars in the vicinity of the Sun, found that stars exhibited differential rotation – stars closer to the center of the galaxy traveled at higher velocities than stars farther away from the center. In his paper published in 1927, he identified the center of the galaxy in constellation Sagittarius within 2° of Shapley’s estimate. The distance to the Galactic Center according to Oort was 10 kpc, which is much less than Shapley’s estimate (Oort 1970).<o:p></o:p></span></span></div><div class="MsoNormal" style="line-height: 19.0pt; margin-bottom: 6.0pt; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Similar studies in the 1970s and 1980s with much better data and absorption corrections yielded half shorter distances about 8 kpc.<o:p></o:p></span></span></div><div class="MsoNormal" style="line-height: 19.0pt; margin-bottom: 6.0pt; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Kapteyn, Shapley and Oort produce such divergent estimates of the distance to the galaxy center because at that time they did not have an important piece of information – the knowledge of interstellar extinction.<o:p></o:p></span></span></div><div class="MsoNormal" style="line-height: 19.0pt; margin-bottom: 6.0pt; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><o:p><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> </span></o:p></span><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><b>References:</b></span></div><div class="MsoNormal" style="line-height: 19.0pt; margin-bottom: 6.0pt; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">HershelWeb - http://en.wikipedia.org/wiki/William_Herschel#Deep_Sky_Surveys<o:p></o:p></span></span></div><div class="MsoNormal" style="line-height: 19.0pt; margin-bottom: 6.0pt; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">KapteinWeb- http://en.wikipedia.org/wiki/Jacobus_Kapteyn<o:p></o:p></span></span></div><div class="MsoNormal" style="line-height: 19.0pt; margin-bottom: 6.0pt; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">CudworthWeb-http://nedwww.ipac.caltech.edu/level5/ESSAYS/Cudworth/cudworth.html<o:p></o:p></span></span></div><div class="MsoNormal" style="line-height: 19.0pt; margin-bottom: 6.0pt; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Oort, JH (1970), "Galaxies and the Universe: Properties of the universe are revealed by the rotation of galaxies and their distribution in space", </span><i><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Science</span></i><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> </span><b><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">170</span></b><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> (3965): 1363–1370, 1970 Dec 25, </span><a href="http://en.wikipedia.org/wiki/Bibcode"><span style="color: #0842b5; text-decoration: none;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Bibcode</span></span></a><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> </span><a href="http://adsabs.harvard.edu/abs/1970Sci...170.1363O"><span style="color: #3464c2; text-decoration: none;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">1970Sci...170.1363O</span></span></a><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">, </span><a href="http://en.wikipedia.org/wiki/Digital_object_identifier"><span style="color: #0842b5; text-decoration: none;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">doi</span></span></a><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">:</span><a href="http://dx.doi.org/10.1126%2Fscience.170.3965.1363"><span style="color: #3464c2; text-decoration: none;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">10.1126/science.170.3965.1363</span></span></a><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">, </span><a href="http://en.wikipedia.org/wiki/PubMed_Identifier"><span style="color: #0842b5; text-decoration: none;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">PMID</span></span></a><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> </span><a href="http://www.ncbi.nlm.nih.gov/pubmed/17817459"><span style="color: #3464c2; text-decoration: none;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">17817459</span></span></a><o:p></o:p></span></div><br />
<span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.jtitle=The+Astronomical+Journal&rft_id=info%3Adoi%2F10.1086%2F114393&rfr_id=info%3Asid%2Fresearchblogging.org&rft.atitle=Milky+Way+rotation+and+the+distance+to+the+galactic+center+from+Cepheid+variables&rft.issn=00046256&rft.date=1987&rft.volume=93&rft.issue=&rft.spage=1090&rft.epage=&rft.artnum=http%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fbib_query%3F1987AJ.....93.1090C&rft.au=Caldwell%2C+J.&rft.au=Coulson%2C+I.&rfe_dat=bpr3.included=1;bpr3.tags=Astronomy">Caldwell, J., & Coulson, I. (1987). Milky Way rotation and the distance to the galactic center from Cepheid variables <span style="font-style: italic;">The Astronomical Journal, 93</span> DOI: <a rev="review" href="http://dx.doi.org/10.1086/114393">10.1086/114393</a></span>Olga V. Vovkhttp://www.blogger.com/profile/10788084870025950079noreply@blogger.com0tag:blogger.com,1999:blog-9016726145636701998.post-82853588123655836602011-04-29T19:03:00.000-07:002011-05-02T17:43:40.971-07:00Milky Way: a Distance to the Galactic Center - 2<h3><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: large;">Why distance to the galactic center is so important?</span></span><o:p></o:p></h3><div class="MsoNormal" style="line-height: 19.0pt; margin-bottom: 6.0pt; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">The distance from Sun and the center of Milky Way is used as a reference stick for many other extragalactic distance calculations, making its accurate determination a matter of extreme importance.<o:p></o:p></span></span></div><div class="MsoNormal" style="line-height: 19.0pt; margin-bottom: 6.0pt; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">The only direct method to determine distances to cosmic objects outside the Solar System is trigonometrical parallax. Ground-based telescopes allow to measure parallaxes up to ~0.01-arcsec, which allowed the distance estimates up to 100 pc. We can observe less than 1000 stars at these distances. The Hipparcos satellite measures parallaxes up to ~0.001 arcsec, which provides good distances out to 1000 pc for about 100,000 stars. After 1000 pc we should employ indirect methods for distance estimates, such as spectroscopic parallaxes and properties of periodic variable stars.<o:p></o:p></span></span></div><div class="MsoNormal" style="line-height: 19.0pt; margin-bottom: 6.0pt; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">No single method can provide accurate distances on all cosmic scales. Instead, we have to rely on a multi-step multi-method approach carefully choosing each method and calibrating at each step. This makes the Cosmic Distance Scale look like a ladder with a series of steps going from near to far. We should<o:p></o:p></span></span></div><div class="MsoNormal" style="line-height: 19.0pt; margin-bottom: 6.0pt; margin-left: 14.2pt; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-list: l0 level1 lfo1; mso-pagination: none; text-autospace: none; text-indent: 0cm;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Calibrate parallaxes based on the orbit of the Earth (the Astronomical Unit);<o:p></o:p></span></span></div><div class="MsoNormal" style="line-height: 19.0pt; margin-bottom: 6.0pt; margin-left: 14.2pt; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-list: l0 level1 lfo1; mso-pagination: none; text-autospace: none; text-indent: 0cm;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Calibrate H-R diagram methods based on distances to the stars with measured parallaxes;<o:p></o:p></span></span></div><div class="MsoNormal" style="line-height: 19.0pt; margin-bottom: 6.0pt; margin-left: 14.2pt; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-list: l0 level1 lfo1; mso-pagination: none; text-autospace: none; text-indent: 0cm;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Calibrate distances to Cepheid and RR Lyrae stars using H-R diagrams;<o:p></o:p></span></span></div><div class="MsoNormal" style="line-height: 19.0pt; margin-bottom: 6.0pt; margin-left: 14.2pt; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-list: l0 level1 lfo1; mso-pagination: none; text-autospace: none; text-indent: 0cm;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Calibrate extragalactic distances based on the known distance to galactic center.<o:p></o:p></span></span><br />
<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><b>So one must calibrate, calibrate and calibrate.</b></span></div><div class="MsoNormal" style="line-height: 19.0pt; margin-bottom: 6.0pt; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Because of the above reason, astronomers have tried to accurately measure the distance to the Galactic Center since it discovery in the early 20th century. Before that people still believed that our Solar System is the center of the Milky Way. <o:p></o:p></span></span></div><div class="MsoNormal" style="line-height: 19.0pt; margin-bottom: 6.0pt; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">While the first estimates provided by Harlow Shapley and Jan Oort were despondently incorrect, recent technological advancements have enabled astronomers to estimate the distance to the galactic center with increasing accuracy.</span><o:p></o:p></span><br />
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<span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">References:</span></span><br />
<span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> </span></span><br />
<span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"></span></span><br />
<span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><div class="MsoNormal">Carretta and Gratton 2000, Distances, Ages, and Epoch of Formation of Globular Clusters, The Astrophysical Journal, 533:215-235<o:p></o:p></div><div class="MsoNormal"><br />
</div><div class="MsoNormal">Carretta, Gratton, Clementini, 2000, Mon. Not. R. Astron. Soc. 316, 721±728 (2000)<o:p></o:p></div></span></span><br />
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<span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><div class="MsoNormal">Popowski and Gould, 1998, <a href="http://arxiv.org/abs/astro-ph/9703140"><u style="text-underline: #0016E7;"><span style="color: windowtext; text-decoration: none;">"Mathematics of Statistical Parallax and the Local Distance Scale"</span></u></a>. arXiv, Ohio State University. Retrieved 2008-10-20.<o:p></o:p></div><div class="MsoNormal"><br />
</div><div class="MsoNormal">Percival, Salaris and Kilkenny, 2003, The open cluster distance scale, DOI: 10.1051/0004-6361:20030092<o:p></o:p></div></span></span><br />
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<span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.jtitle=Astronomy+and+Astrophysics&rft_id=info%3Adoi%2F10.1051%2F0004-6361%3A20030092&rfr_id=info%3Asid%2Fresearchblogging.org&rft.atitle=Why+distance+to+the+galactic+center+is+so+important%3F&rft.issn=0004-6361&rft.date=2003&rft.volume=400&rft.issue=2&rft.spage=541&rft.epage=552&rft.artnum=http%3A%2F%2Fwww.edpsciences.org%2F10.1051%2F0004-6361%3A20030092&rft.au=Percival%2C+S.&rft.au=Salaris%2C+M.&rft.au=Kilkenny%2C+D.&rfe_dat=bpr3.included=1;bpr3.tags=Astronomy">Percival, S., Salaris, M., & Kilkenny, D. (2003). Why distance to the galactic center is so important? <span style="font-style: italic;">Astronomy and Astrophysics, 400</span> (2), 541-552 DOI: <a rev="review" href="http://dx.doi.org/10.1051/0004-6361:20030092">10.1051/0004-6361:20030092</a></span>Olga V. Vovkhttp://www.blogger.com/profile/10788084870025950079noreply@blogger.com0tag:blogger.com,1999:blog-9016726145636701998.post-27307568039577307242011-04-27T14:15:00.000-07:002011-04-27T14:30:26.225-07:00Milky Way: Distance to the Galactic Centre<div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhT5AAWiqWn4VY0JdIt_qYGPbAaBqEiq8y5HshGg7dDpmaTKg2i0i5z5Ijn32DZ2gGGYuhyxT_AGQUnSyuPaj2pwmZRQYPz31dJplRWy0hJf0QVZ-HOUT-6B2uIeZzm5hUCOKM4wjNSs8fW/s1600/MilkyWay1.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="30" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhT5AAWiqWn4VY0JdIt_qYGPbAaBqEiq8y5HshGg7dDpmaTKg2i0i5z5Ijn32DZ2gGGYuhyxT_AGQUnSyuPaj2pwmZRQYPz31dJplRWy0hJf0QVZ-HOUT-6B2uIeZzm5hUCOKM4wjNSs8fW/s320/MilkyWay1.jpg" width="320" /></a></div><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br />
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<div class="MsoNormal" style="line-height: 19.0pt; margin-bottom: 6.0pt; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><div style="text-align: center;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-family: Times; line-height: normal;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: x-small;">A reprocessed cropped portion of the 2MASS mosaic of the Milky Way (Cutri </span></span></span><span class="Apple-style-span" style="line-height: normal;"><i><span class="Apple-style-span" style="font-size: x-small;">et al. </span></i></span><span class="Apple-style-span" style="line-height: normal;"><span class="Apple-style-span" style="font-size: x-small;">2003). The Galactic bulge exhibits a peanut-like morphology.</span></span></span></span></div><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">The Galactic Center cannot be studied at visible, ultraviolet or soft X-ray wavelengths, because of the interstellar dust that hides it from observation. The available information comes from observations at radio, infrared, sub- millimeter and hard X-rays.<o:p></o:p></span></span><br />
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<span class="Apple-style-span" style="line-height: normal;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">The main problem in estimating the distance to the Galactic Center is a proper calculation of </span></span><span class="Apple-style-span" style="line-height: normal;"><i><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">extinction</span></i></span><span class="Apple-style-span" style="line-height: normal;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">. </span></span><br />
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<span style="color: windowtext;"><div style="line-height: normal; margin-bottom: 0px; margin-left: 0px; margin-right: 0px; margin-top: 0px;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br />
</span></div><div class="MsoNormal" style="line-height: 19pt; margin-bottom: 6pt; margin-left: 0cm; margin-right: 0cm; margin-top: 0cm;"><div style="margin-bottom: 0px; margin-left: 0px; margin-right: 0px; margin-top: 0px;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Extinction is the dimming of light from stars and other distant objects, due the combined effects of </span><a href="http://www.daviddarling.info/encyclopedia/I/interstellar_absorption.html"><span style="color: windowtext; text-decoration: none;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">interstellar absorption</span></span></a><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> and scattering of light by </span><a href="http://www.daviddarling.info/encyclopedia/C/cosmicdust.html"><span style="color: windowtext; text-decoration: none;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">dust</span></span></a><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> particles. Interstellar extinction increases at shorter wavelengths, resulting in </span><a href="http://www.daviddarling.info/encyclopedia/I/interstellar_reddening.html"><span style="color: windowtext; text-decoration: none;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">interstellar reddening</span></span></a><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">. Extinction is minor in longer wavelengths - radio and infrared, which makes them more suitable for observing at large distances in the galactic.</span></span><br />
<div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj1gU0QPUPnGLqf-KkLC3iPx8qvqqeIi9mFVByp8UsHBxl4IWl0SRDUiyav4hm606mNQBoc0O1uyWT0Er-jGMtF5QSrk3BnZapK4PWh13LBtVG6T30KHl0pd3SQhUDPkRmo29cPgIpFK0ql/s1600/Galactic_Cntr_full_cropped.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj1gU0QPUPnGLqf-KkLC3iPx8qvqqeIi9mFVByp8UsHBxl4IWl0SRDUiyav4hm606mNQBoc0O1uyWT0Er-jGMtF5QSrk3BnZapK4PWh13LBtVG6T30KHl0pd3SQhUDPkRmo29cPgIpFK0ql/s320/Galactic_Cntr_full_cropped.jpg" width="245" /></a></div></div></div></span></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhT5AAWiqWn4VY0JdIt_qYGPbAaBqEiq8y5HshGg7dDpmaTKg2i0i5z5Ijn32DZ2gGGYuhyxT_AGQUnSyuPaj2pwmZRQYPz31dJplRWy0hJf0QVZ-HOUT-6B2uIeZzm5hUCOKM4wjNSs8fW/s1600/MilkyWay1.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><span class="Apple-style-span" style="-webkit-border-horizontal-spacing: 2px; -webkit-border-vertical-spacing: 2px; color: black; line-height: 18px;"><span class="Apple-style-span" style="font-size: x-small;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">The center of our Milky Way Galaxy is located in the constellation of Sagittarius. The most of the stars are hidden behind thick clouds of dust if observed in visible light, however, this dust becomes transparent at infrared wavelengths. This 2MASS IR image reveals multitudes of otherwise hidden stars, penetrating all the way to the central star cluster of the Galaxy. </span></span></span><span class="Apple-style-span" style="-webkit-border-horizontal-spacing: 2px; -webkit-border-vertical-spacing: 2px; line-height: 18px;"><span class="Apple-style-span" style="-webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; line-height: 25px;"><span class="Apple-style-span" style="-webkit-border-horizontal-spacing: 2px; -webkit-border-vertical-spacing: 2px; line-height: 18px;"><span class="Apple-style-span" style="font-size: x-small;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="color: black;">The reddening of the stars here and along the Galactic Plane is due to dust scattering. </span></span></span></span><span class="Apple-style-span" style="font-size: x-small;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="color: black;">Picture credit: Wikipedia.</span></span></span></span></span></a></div><div class="separator" style="clear: both; text-align: center;"></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br />
</span></div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"> </span></div><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Harlow Shapley first established coordinates of the Galactic Center in 1918. He derived distances for many globular clusters (GC), and found that the distribution of GCs was centered at about 15 kpc away from the Sun in the direction of the constellation Sagittarius. Shapley derived his cluster distances based on the brightnesses of individual stars in a cluster when possible, and for</span></span><span style="color: #000087;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> </span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">those clusters where individual stars could not be resolved, on the size and brightness of each cluster as a whole.</span></span><br />
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<div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Because many of the GCs, which Shapley studied, are out of the dusty Galaxy plane, the distances that he found were not too severely affected by extinction. <o:p></o:p></span></span></div><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br />
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<span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">The Shapley’s main argument was that such massive objects as GCs are most likely to be centered on the galactic center. However Shapley's conclusions remained controversial at the beginning, they were eventually accepted by majority of astronomers, and his technique is still considered one of the primary means of determining the distance to the center of the Galaxy.<o:p></o:p></span></span></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhi8iRmGfUZbzhH_Djt5j3LSBBiKWeZjqrV19YN16KkpodG52jVNHYsaOdft24KoSD3DRiG7nxIoCptj1fksissNUNyQe6fXEek7G5rkE-VOdj8lNE6DOlAuMwEC9Ph119sbWw5Lmar3zHy/s1600/GCdistribution.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><img border="0" height="241" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhi8iRmGfUZbzhH_Djt5j3LSBBiKWeZjqrV19YN16KkpodG52jVNHYsaOdft24KoSD3DRiG7nxIoCptj1fksissNUNyQe6fXEek7G5rkE-VOdj8lNE6DOlAuMwEC9Ph119sbWw5Lmar3zHy/s320/GCdistribution.jpg" width="320" /></span></a></div><div class="separator" style="clear: both; text-align: center;"><span class="Apple-style-span" style="font-size: x-small;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Picture credit:</span></span></div><div class="MsoNormal" style="text-align: center;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: x-small;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">http://www.phys.boun.edu.tr/~semiz/universe/far/13.html</span></span></span><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> </span></div><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Similar studies in the 1970s and 1980s with much better data and absorption corrections yielded half shorter distances - 8 instead of 15 kpc.<o:p></o:p></span></span></div><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br />
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<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">The exact distance from the Earth to the Galactic Center is still uncertain. </span><span class="Apple-style-span" style="line-height: 25px;"><b><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">The latest estimates based on both - geometric-based methods and standard candles produce distances of 7.6–8.7 kpc with more than 1Kpc uncertainty.</span></b></span><br />
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<b><div class="MsoNormal" style="display: inline !important;"><div style="display: inline !important;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-weight: normal;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br />
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<b><div class="MsoNormal" style="display: inline !important;"><div style="display: inline !important;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-weight: normal;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Eisenhauer, F. et al. (2005) used geometrical-based method combined with near IR imaging spectroscopy (with astrometric accuracy of 75 mas) to observe the central 30 light-days close to the Galactic Center. They determined radial velocities for 9 of 10 stars in the central 0.4”, and for 13 of 17 stars out to 0.7”, limiting stars magnitudes to K~16. They combined the calculated radial velocities with astrometrical data, and then used a global fit technique to derive new improved three-dimensional stellar orbits for 6 S stars in the central 0.5” region. This result in the updated estimate for the distance to the Galactic Center Ro= 7,62 +-0,32 kpc.</span></span></span></div></div></b><br />
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<span class="Apple-style-span" style="font-family: Arial, Helvetica, sans-serif;"><b><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-weight: normal;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br />
</span></span></span></div><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-weight: normal;"><span class="Apple-style-span" style="font-size: x-small;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">The instrumentation they used is SINFONI - a near-infrared (1.1 - 2.45 µm) integral field spectrograph connected to an adaptive optics module, installed on ESO VLT. The instrument operates with 4 gratings (J, H, K, H+K) providing a spectral resolution around 2000, 3000, 4000 in J, H, K, respectively, and 1500 in H+K. For more information about SINFONI, please refer to the following page: </span></span></span><a href="http://www.eso.org/sci/facilities/paranal/instruments/sinfoni/overview.html"><span style="color: black; text-decoration: none;"><span class="Apple-style-span" style="font-weight: normal;"><span class="Apple-style-span" style="font-size: x-small;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">http://www.eso.org/sci/facilities/paranal/instruments/sinfoni/overview.html</span></span></span></span></a><span class="Apple-style-span" style="font-weight: normal;"><o:p></o:p></span></span></div><div class="MsoNormal"><br />
</div><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none;"><span style="color: windowtext;"><span class="Apple-style-span" style="font-weight: normal;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Vanhollebeke, Groenewegen, and Girardi (2009) employed the different approach. They used the star population synthesis code called TRILEGAL (TRIdimensional modeL of thE GALaxy, Girardi et al. 2005) to compute colour-magnitude diagrams (CMD) towards the galactic bulge (GB) and Galactic Center. They simulated the photometric properties of stars located towards a given direction and limited simulations to given magnitudes. The simulations were run for several star formation rates and metallicity distributions. </span></span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-weight: normal;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Extinction was calculated for each object separately based on its visual extinction value and the distance modulus of the object. </span></span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-weight: normal;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Based on their simulations, the distance to the Galactic Centre was determined as R</span></span><sub><span class="Apple-style-span" style="font-weight: normal;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: small;">0</span></span></span></sub><span class="Apple-style-span" style="font-weight: normal;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> = 8.7+-0.57 - 0.43 kpc.</span><o:p></o:p></span></span></div><div class="MsoNormal"><br />
</div><span class="Apple-style-span" style="font-weight: normal;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Majaess (2010) used sample RR Lyrae variables from the OGLE survey of Galactic bulge fields to estimate R</span></span><sub><span class="Apple-style-span" style="font-weight: normal;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: small;">0</span></span></span></sub><span class="Apple-style-span" style="font-weight: normal;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> using the standard candles method. Majaess (2010) paid a special attention to the effects that can bias the accurate measurements of R</span></span><sub><span class="Apple-style-span" style="font-weight: normal;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: small;">0</span></span></span></sub><span class="Apple-style-span" style="font-weight: normal;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">. These include a) an ambiguous extinction, which in turn imposes a preferential sampling of stars toward the near side of the GB, resulting in a smaller mean value of R</span></span><sub><span class="Apple-style-span" style="font-weight: normal;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: small;">0</span></span></span></sub><span class="Apple-style-span" style="font-weight: normal;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">, and b) an uncertainty in characterizing how a mean distance to the group of variable stars relates to R</span></span><sub><span class="Apple-style-span" style="font-weight: normal;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: small;">0</span></span></span></sub><span class="Apple-style-span" style="font-weight: normal;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">. The result is R</span></span><sub><span class="Apple-style-span" style="font-weight: normal;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><span class="Apple-style-span" style="font-size: small;">0</span></span></span></sub><span class="Apple-style-span" style="font-weight: normal;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">=8.1+-0.6 kpc.</span></span> </b></span><br />
<div><span class="Apple-style-span" style="font-family: Arial, Helvetica, sans-serif;"><b><span class="Apple-style-span" style="font-weight: normal;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"> </span></span></b></span><br />
<span class="Apple-style-span" style="font-family: Arial, Helvetica, sans-serif;"><b><span class="Apple-style-span" style="font-weight: normal;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"></span></span></b></span><br />
<span class="Apple-style-span" style="font-family: Arial, Helvetica, sans-serif;"><b><span class="Apple-style-span" style="font-weight: normal;"><span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><div class="MsoNormal"><span style="color: windowtext; font-family: Helvetica; font-size: 13pt;"><b>References:</b><o:p></o:p></span></div><div class="MsoNormal"><span style="color: windowtext;"><a href="http://nedwww.ipac.caltech.edu/level5/ESSAYS/Cudworth/cudworth.html"><span class="Apple-style-span" style="font-size: x-small;">http://nedwww.ipac.caltech.edu/level5/ESSAYS/Cudworth/cudworth.html</span></a><span class="Apple-style-span" style="font-size: x-small;"><o:p></o:p></span></span></div><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: x-small;">http://www.phys.boun.edu.tr/~semiz/universe/far/13.html<o:p></o:p></span></span></div><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: x-small;">Eisenhauer, F. et al. 2005, </span><a href="http://adsabs.harvard.edu/abs/2005ApJ...628..246E"><span style="color: #3464c2; text-decoration: none;"><span class="Apple-style-span" style="font-size: x-small;">"SINFONI in the Galactic Center: Young Stars and Infrared Flares in the Central Light-Month"</span></span></a><span class="Apple-style-span" style="font-size: x-small;">. </span><i><span class="Apple-style-span" style="font-size: x-small;">AJ</span></i><span class="Apple-style-span" style="font-size: x-small;">. 628</span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: x-small;"><o:p></o:p></span></span></div><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: x-small;">Vanhollebeke E., Groenewegen M. A. T., Girardi L.. "Stellar populations in the Galactic bulge. Modelling the Galactic bulge with TRILEGAL". </span><i><span class="Apple-style-span" style="font-size: x-small;">A&A</span></i><span class="Apple-style-span" style="font-size: x-small;"> </span><b><span class="Apple-style-span" style="font-size: x-small;">498</span></b><span class="Apple-style-span" style="font-size: x-small;">: 2009. </span><a href="http://en.wikipedia.org/wiki/Bibcode"><span style="color: #0842b5; text-decoration: none;"><span class="Apple-style-span" style="font-size: x-small;">Bibcode</span></span></a><span class="Apple-style-span" style="font-size: x-small;"> </span><a href="http://adsabs.harvard.edu/abs/2009A&A...498...95V"><span style="color: #3464c2; text-decoration: none;"><span class="Apple-style-span" style="font-size: x-small;">2009A&A...498...95V</span></span></a><span class="Apple-style-span" style="font-size: x-small;">.</span></span><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: x-small;"><o:p></o:p></span></span></div><div class="MsoNormal"><span style="color: windowtext;"><span class="Apple-style-span" style="font-size: x-small;">Majaess, D.</span><a href="http://adsabs.harvard.edu/abs/2010AcA....60...55M"><span style="color: #3464c2; text-decoration: none;"><span class="Apple-style-span" style="font-size: x-small;">"Concerning the Distance to the Center of the Milky Way and Its Structure"</span></span></a><span class="Apple-style-span" style="font-size: x-small;">. </span><i><span class="Apple-style-span" style="font-size: x-small;">Acta A.</span></i><span class="Apple-style-span" style="font-size: x-small;">. 60 (2010)<o:p></o:p></span></span></div><span class="Apple-style-span" style="font-size: x-small;">Cutri R.M., et al. 2003, The IRSA 2MASS All-Sky Point Source Catalog of Point Sources, NASA/IPAC, Infrared Science Archive.</span> </span></span></b></span></div>Olga V. Vovkhttp://www.blogger.com/profile/10788084870025950079noreply@blogger.com0tag:blogger.com,1999:blog-9016726145636701998.post-23602997518127309772011-04-18T13:59:00.000-07:002011-04-18T14:53:22.444-07:00Celestial events in April 2011I made this list of interesting celestial events using the information from the NASA web site and also from the Goddard Astronomy Club mailings. I would like to say thank you to people who are doing such a great job by gathering this information.<br />
<br />
So here it is:<br />
On Monday, April 18, evening, the International Space Station (ISS) will fly high above the National Capital area and will be visible with naked eyes (if weather permits). It is going to be the brightest object in the sky, except for the Moon, which will be extremely low in the east-southeast.<br />
<br />
According to the PRESS RELEASE from National Capital Astronomers:<br />
<br />
"ISS will rise in the West about 9:16 pm EDT, moving up and to the right,<br />
into the heart of the bright stars of the winter sky. About 2 minutes later, she will be flying about 6 degrees right of the bright star Betelgeuse, being about 27 degrees altitude over azimuth 263 degrees. About 91 seconds later, ISS will culminate fairly high in the northwest at about 49 degrees over 326.<br />
About 27 seconds later, she will be due North, about 4 degrees above<br />
the North Star, Polaris. About 77 seconds later, ISS will disappear into the shadow of the Earth in the northeast, being about 20 degrees above 35 degrees."<br />
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<br />
<span class="date-text" style="font-weight: bold;"><span class="Apple-style-span" style="color: black;">April 18</span></span><span class="Apple-style-span" style="color: black;"> - a f</span><span class="title-text"><span class="Apple-style-span" style="color: black;">ull Moon.</span></span><span class="Apple-style-span" style="color: black;"> The Moon will be directly opposite the Earth from the Sun and will be fully illuminated as seen from Earth. This type of the full Moon was known by early Native American tribes as the Full Pink Moon . This year, it is also known as the Paschal Full Moon because it is the first full moon of the spring season.</span><br />
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<span class="date-text" style="font-weight: bold;"><span class="Apple-style-span" style="color: black;">April 21, 22 through 25</span></span><span class="Apple-style-span" style="color: black;"> - </span><span class="title-text"><span class="Apple-style-span" style="color: black;">Lyrids Meteor Shower from the constellation Lyra.</span></span><span class="Apple-style-span" style="color: black;"> The Lyrids are usually producing about 20 meteors per hour at their peak. However, this year, the gibbous moon will hide most of the fainter meteors in its glare. Look for meteors radiating from the constellation of Lyra after midnight, and be sure to find a dark viewing location far from city lights.</span>Olga V. Vovkhttp://www.blogger.com/profile/10788084870025950079noreply@blogger.com0tag:blogger.com,1999:blog-9016726145636701998.post-72098641968999722932011-04-11T14:30:00.000-07:002011-05-05T19:04:16.062-07:00Measuring stellar distances for stars more than 1000 pc away<div class="MsoNormal">This post was inspired by my colleague SAO student, who asked on how we can measure stellar distances for stars which are so far away that their parallaxes are not measurable yet.</div><div class="MsoNormal">For stars more than 1000 pc away parallaxes are not measurable yet, so the only way to estimate distances is using photometry. The connection between star brightness and its distance is known as the <i>inverse-square-law</i> (Equation 1.<i></i>).</div><div class="MsoCaption"><a href="http://www.blogger.com/post-create.g?blogID=9016726145636701998" name="_Ref162616819">Equation </a>1.</div><div class="MsoNormal"><span style="font-family: Verdana;"><span class="Apple-style-span" style="font-size: x-small;">B=(L/2Pid</span><sup><span class="Apple-style-span" style="font-size: x-small;">2</span></sup><span class="Apple-style-span" style="font-size: x-small;">)<o:p></o:p></span></span></div><div class="MsoNormal"><span style="font-family: Verdana;"><span class="Apple-style-span" style="font-size: x-small;">dL=(L/4PiB)</span><sup><span class="Apple-style-span" style="font-size: x-small;">1/2</span></sup><span class="Apple-style-span" style="font-size: x-small;"><o:p></o:p></span></span></div><div class="MsoNormal" style="margin-left: 14.2pt; text-indent: 0cm;"><span style="font-family: Verdana;"><span class="Apple-style-span" style="font-size: x-small;">Where, B is the star’s apparent brightness, L is its luminosity (W m</span></span><sup><span style="font-family: Verdana;"><span class="Apple-style-span" style="font-size: x-small;">-2</span></span></sup><span style="font-family: Verdana;"><span class="Apple-style-span" style="font-size: x-small;">), d is its distance to an observer.</span><o:p></o:p></span></div><div class="MsoNormal">Equation 1. needs to be changed if one takes into account the expansion of the Universe, but this is irrelevant for all stars in our Local Group. </div><div class="MsoNormal">We can measure the star’s apparent brightness B; then assume the star’s luminosity L; and then solve Equation 1. for the object's luminosity distance dL. The problem is that we can only measure star’s brightness B and its apparent magnitude m, while the luminosity and absolute magnitude must be derived in some way.</div><div class="MsoNormal">There are two methods for deriving luminosities – one uses spectroscopic parallaxes and another uses standard candles.<a href="http://www.blogger.com/post-create.g?blogID=9016726145636701998#_ftn1" name="_ftnref" style="mso-footnote-id: ftn;" title=""><span class="MsoFootnoteReference">[1]</span></a> Both methods use distant-independent properties of stars, which make them good techniques for distance measurements.</div><div class="MsoNormal">A distance-independent property is the property of star, which doesn’t depend on its distance from an observer. For example, a period for a variable star is a distant-independent property. Star’s spectrum can serve as a distant-independent property. The later, however, is not fully distance-independent and works well only up to 100 000 pc given the current technology level.</div><h3>Spectroscopic parallaxes</h3><div class="MsoNormal">The spectroscopic parallax method uses the observed spectrum of the star as a distance-independent property to derive its luminosity (Figure 1). It involves the following steps:</div><div class="MsoListParagraphCxSpFirst" style="margin-left: 42.55pt; mso-add-space: auto; mso-list: l0 level1 lfo12; text-indent: -32.2pt;">1.<span style="font: normal normal normal 7pt/normal 'Times New Roman';"> </span>Choose a star of interest;</div><div class="MsoListParagraphCxSpMiddle" style="margin-left: 42.55pt; mso-add-space: auto; mso-list: l0 level1 lfo12; text-indent: -32.2pt;">2.<span style="font: normal normal normal 7pt/normal 'Times New Roman';"> </span>Plot a calibrated Hertzsprung–Russell (H-R) diagram for nearby stars with known parallax distances;</div><div class="MsoListParagraphCxSpMiddle" style="margin-left: 42.55pt; mso-add-space: auto; mso-list: l0 level1 lfo12; text-indent: -32.2pt;">3.<span style="font: normal normal normal 7pt/normal 'Times New Roman';"> </span>Observe spectrum of the star of interest;</div><div class="MsoListParagraphCxSpMiddle" style="margin-left: 35.45pt; mso-add-space: auto; mso-list: l0 level1 lfo12; text-indent: -25.1pt;">4.<span style="font: normal normal normal 7pt/normal 'Times New Roman';"> </span>The spectral type (OBAFGKML) and luminosity class (I-V) provide the star’s unique location on a calibrated H-R diagram;</div><div class="MsoListParagraphCxSpMiddle" style="margin-left: 35.45pt; mso-add-space: auto; mso-list: l0 level1 lfo12; text-indent: -25.1pt;">5.<span style="font: normal normal normal 7pt/normal 'Times New Roman';"> </span>Once the star’s position on the H-R diagram is known, we can deduce its absolute magnitude (M).</div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgZgAF92cwgQ4MLIFTTiWPh-3NfAVv6-59_8hzSzASyF3faAa_kGrC41rIdkXZYSR6sy0dX6c06J9XCsYB0PMBMLilwtwn-2i_19BAbWIBcwrOLfE7iGBAoFEfwDUPduhjac8Ww4p-8LAW8/s1600/SpecParallax.gif" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="300" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgZgAF92cwgQ4MLIFTTiWPh-3NfAVv6-59_8hzSzASyF3faAa_kGrC41rIdkXZYSR6sy0dX6c06J9XCsYB0PMBMLilwtwn-2i_19BAbWIBcwrOLfE7iGBAoFEfwDUPduhjac8Ww4p-8LAW8/s400/SpecParallax.gif" width="400" /></a></div><div class="MsoListParagraphCxSpLast" style="margin-left: 0cm; mso-add-space: auto; text-indent: 0cm;"><o:p></o:p></div><div class="MsoCaption"><a href="http://www.blogger.com/post-create.g?blogID=9016726145636701998" name="_Ref162617585"><span class="Apple-style-span" style="font-size: x-small;">Figure </span></a><span class="Apple-style-span" style="font-size: x-small;">1. Spectroscopic parallax. </span><i><span class="Apple-style-span" style="font-size: x-small;">Image, courtesy of Prof. Richard Pogge, Ohio State Univ.</span></i></div><div class="MsoNormal"><span class="Apple-style-span" style="font-size: x-small;">As soon as we derived m from measurements and M from the H-R diagram, we can use the distance modulus (Equation 2.) to find the distance to the star - d, in parsecs.</span></div><div class="MsoCaption"><a href="http://www.blogger.com/post-create.g?blogID=9016726145636701998" name="_Ref162617746">Equation </a>2.</div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgymKqvdpaikd9v0HL3KgdqFtFPrgcCJdWDNmTxdG7YiQ2TQ7Ui-Tky_963by3YC6qfM_vLMgDnR8ctCxh68mzteQKWn9BqTd5e_VJCyABeqtWi6KKaO7nasbLN4Lfa14_z0cudJU3vRY31/s1600/distancemodulus.gif" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="76" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgymKqvdpaikd9v0HL3KgdqFtFPrgcCJdWDNmTxdG7YiQ2TQ7Ui-Tky_963by3YC6qfM_vLMgDnR8ctCxh68mzteQKWn9BqTd5e_VJCyABeqtWi6KKaO7nasbLN4Lfa14_z0cudJU3vRY31/s200/distancemodulus.gif" width="200" /></a></div><div class="MsoNormal"><o:p></o:p></div><div class="MsoNormal">However, the spectroscopic parallax method suffers uncertainties, which range from about 0.7 up to 1.25 absolute magnitudes, which in turn gives a factor of 1.4 to 1.8 variation in the distance (ScienceVaultWeb 2010): </div><div class="MsoListParagraphCxSpFirst" style="margin-left: 50.2pt; mso-add-space: auto; mso-list: l7 level1 lfo9; text-indent: -18.0pt;"><span style="font-family: Symbol;">·<span style="font: normal normal normal 7pt/normal 'Times New Roman';"> </span></span>The assumption was made that the spectra from distant stars of interest are the same as spectra from nearby stars. <o:p></o:p></div><div class="MsoListParagraphCxSpMiddle" style="margin-left: 50.2pt; mso-add-space: auto; mso-list: l7 level1 lfo9; text-indent: -18.0pt;"><span style="font-family: Symbol;">·<span style="font: normal normal normal 7pt/normal 'Times New Roman';"> </span></span>Due to difficulties in observing spectra, it works up to 100 000 pc only. <o:p></o:p></div><div class="MsoListParagraphCxSpMiddle" style="margin-left: 50.2pt; mso-add-space: auto; mso-list: l7 level1 lfo9; text-indent: -18.0pt;"><span style="font-family: Symbol;">·<span style="font: normal normal normal 7pt/normal 'Times New Roman';"> </span></span>Interstellar dust can scatter the different frequencies in different ways, making the identification of spectral class even harder.<o:p></o:p></div><div class="MsoListParagraphCxSpMiddle" style="margin-left: 50.2pt; mso-add-space: auto; mso-list: l7 level1 lfo9; text-indent: -18.0pt;"><span style="color: #333333; font-family: Symbol;">·<span style="font: normal normal normal 7pt/normal 'Times New Roman';"> </span></span>Matter between the star and the observer would absorb some of the light and make the star's apparent brightness less than it should be<span style="color: #333333; font-family: Arial;">.<o:p></o:p></span></div><div class="MsoListParagraphCxSpLast" style="margin-left: 50.2pt; mso-add-space: auto; mso-list: l7 level1 lfo9; text-indent: -18.0pt;"><span style="font-family: Symbol;">·<span style="font: normal normal normal 7pt/normal 'Times New Roman';"> </span></span>The star’s location on H-R diagram depends on its composition and luminosity class, which is difficult to determine for distant stars due to faint spectra. And there are many stars, which don't belong to the Main Sequence.<o:p></o:p></div><div class="MsoNormal">Spectroscopic parallaxes, however, work well for star clusters, where one can average out many measurements. Since all of the stars in the cluster are the same distance away from us, all of them will have an equal displacement along the Luminosity axis on H-R diagram. Therefore, the cluster's Main Sequence will appear to be shifted vertically in the H-R diagram from the nearby stars. Now, we can measure how much we have to shift the entire set of nearby stars so that they overlap the Main Sequence of the cluster, and can estimate how far away the cluster is.<o:p></o:p></div><div class="MsoNormal">The distances to the closest clusters, e.g. Hyades (46.34 pc), Pleiades (135 pc) can be measured directly using the parallax method (Perryman et al. 1998, HipparcosESAWeb). Once nearby open cluster distances are determined, they can be used to estimate the distances to other open clusters, up to 10—15 kpc away. Once the distances to Pleiades is determined, the distance to a Cepheid Variables can be calibrated (Popowski and Gould 1998; Percival, Salaris and Kilkenny, 2003).<o:p></o:p></div><h3>This is important: since the only directly measurable distances in astronomy are those made by trigonometric parallax, the results from other techniques should be calibrated using parallax data.</h3><h3>Cepheids and RR Lyrae stars as standard candles</h3><div class="MsoNormal">Cepheids and RR Lyrae are variable stars whose brightness varies regularly with a characteristic, periodic pattern. Period of their brightness variations is a distance-independent property and can be used for estimating distances to these stars.<o:p></o:p></div><div class="MsoNormal">Cepheids are located in the upper part of the instability strip in the H-R diagram, while RR Lyrae in the lower part. Most massive stars enter the instability strip and become variable after they have left the main sequence (Figure 2.)<o:p></o:p></div><div class="MsoNormal">In 1912 Henrietta Leavitt (1868—1921) published the results of her study of variable stars in the Large and Small Magellanic Clouds. She found that the fainter Cepheids have shorter pulsating periods. Because all Cepheids in a Magellanic Cloud are at the same distance from us, Leavitt concluded that the more luminous Cepheids pulsated more slowly.<o:p></o:p></div><div class="MsoNormal">In 1950's astronomers found that there are actually two types of Cepheids:<o:p></o:p></div><div class="MsoNormal" style="margin-left: 50.2pt; mso-list: l8 level1 lfo13; text-indent: -18.0pt;">1.<span style="font: normal normal normal 7pt/normal 'Times New Roman';"> </span>Type I or classical Cepheids are from young high-metallicity stars and are about 4 times more luminous than Type II;<o:p></o:p></div><div class="MsoNormal" style="margin-left: 50.2pt; mso-list: l8 level1 lfo13; text-indent: -18.0pt;">2.<span style="font: normal normal normal 7pt/normal 'Times New Roman';"> </span>Type II or W Virginis Cepheids.<o:p></o:p></div><div class="MsoNormal">Type I Cepheids are young high-metallicity stars 4 times more luminous than Type II Cepheids. Application of the classical Cepheid P-L relation to W Virginis Cepheids may lead and did lead to a large overestimation of the distances.<o:p></o:p></div><div class="MsoNormal"><br />
</div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjLZSGurI-gaxUe_GqFc-YcAGa3w6r_wusBNMkKZapW6HB0pi9q5K-UPLqUV_B2fnLJ4ecvQBAOn9_q8pokONCNWZqkPMQxudEuIj00RqB0UTGAg1TozRmtkA1rbMyeqNfpVJdYadd80GpU/s1600/instabstrip.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjLZSGurI-gaxUe_GqFc-YcAGa3w6r_wusBNMkKZapW6HB0pi9q5K-UPLqUV_B2fnLJ4ecvQBAOn9_q8pokONCNWZqkPMQxudEuIj00RqB0UTGAg1TozRmtkA1rbMyeqNfpVJdYadd80GpU/s320/instabstrip.jpg" width="283" /></a></div><div class="MsoNormal"><o:p></o:p></div><div class="Table" style="text-align: justify;"><a href="http://www.blogger.com/post-create.g?blogID=9016726145636701998" name="_Ref162618369"><span class="Apple-style-span" style="font-size: x-small;">Figure </span></a><span class="Apple-style-span" style="font-size: x-small;">2. Cepheids and RR Lyrae on H-R diagram – the instability strip. Cepheids and RR Lyrae are variable stars whose brightness varies regularly with a characteristic, periodic pattern. Period of their brightness variations is a distance-independent property and can be used for estimating distances to these stars. </span><b><span class="Apple-style-span" style="font-size: x-small;">Cepheids</span></b><span class="Apple-style-span" style="font-size: x-small;"> are located in the upper part of the instability strip in the H-R diagram, while </span><b><span class="Apple-style-span" style="font-size: x-small;">RR Lyrae</span></b><span class="Apple-style-span" style="font-size: x-small;"> in the lower part. Most massive stars enter the instability strip and become variable after they have left the main sequence. Figure credit: ScienceVaultWeb.</span></div><h3>How this method works:</h3><div class="MsoNormal" style="margin-left: 50.2pt; mso-list: l5 level1 lfo14; text-indent: -18.0pt;">1.<span style="font: normal normal normal 7pt/normal 'Times New Roman';"> </span>Photometric observations provide the apparent magnitude values for the variable star.<o:p></o:p></div><div class="MsoNormal" style="margin-left: 50.2pt; mso-list: l5 level1 lfo14; text-indent: -18.0pt;">2.<span style="font: normal normal normal 7pt/normal 'Times New Roman';"> </span>Plotting apparent magnitude values from observations at different times against phase (or time) creates a light curve.<o:p></o:p></div><div class="MsoNormal" style="margin-left: 50.2pt; mso-list: l5 level1 lfo14; text-indent: -18.0pt;">3.<span style="font: normal normal normal 7pt/normal 'Times New Roman';"> </span>From the light curve and the photometric data the average apparent magnitude, m, of the star and its period in days can be obtained.<o:p></o:p></div><div class="MsoNormal" style="margin-left: 50.2pt; mso-list: l5 level1 lfo14; text-indent: -18.0pt;">4.<span style="font: normal normal normal 7pt/normal 'Times New Roman';"> </span>The mean absolute magnitude, M, can be obtained by interpolating on the period-luminosity plot. <o:p></o:p></div><div class="MsoNormal" style="margin-left: 50.2pt; mso-list: l5 level1 lfo14; text-indent: -18.0pt;">5.<span style="font: normal normal normal 7pt/normal 'Times New Roman';"> </span>Once apparent magnitude, <i>m</i>, and absolute magnitude, <i>M</i> are known we can simply substitute in to the distance-modulus to obtain the distance to the star.</div><div class="MsoNormal">The question arises “How precise we can measure the period-luminosity (P-L) relation for a given star?”</div><h3>Baade-Wesselink Method</h3><div class="MsoNormal" style="margin-bottom: .0001pt; margin: 0cm; mso-layout-grid-align: none; mso-pagination: none; text-autospace: none; text-indent: 0cm;">The classical method that derives the distance of a pulsating star by comparing its linear radius variation, estimated from the radial velocity curve, with its angular radius variation, which can be estimated from the light curve. The method is named after Walter <a href="http://www.daviddarling.info/encyclopedia/B/Baade.html"><span style="text-decoration: none;">Baade</span></a> and the Dutch astronomer Adriaan Jan Wesselink (1909–1995).<o:p></o:p></div><div class="MsoNormal">The recent advances in interferometry and observational techniques allowed the direct detection of pulsations in nearby Cepheids and RR Lyrae, providing an independent distance measurement of pulsating stars (Cox 1980, Marengo et al. 2004).</div><h3>Issues with using distance measurement techniques for Cepheids and RR Lyrae</h3><div class="MsoNormal">Studies show that in estimating distances using P-L relation an individual Cepheid could deviate up to ± 0.6 mag in B. Such an error if applied to an individual star would end up into an error of about 30% in distance (CaltechCepheidsWeb 2010). Therefore, large samples needed to decrease the error. </div><div class="MsoNormal">The error on the apparent modulus decreases inversely with the square root of the number of samples, so decreasing an error from 30% to 10% is possible with a sample containing a dozen Cepheids.</div><div class="MsoNormal">The other issues include the systematic effects of reddening and the systematic effects of metallicity,. </div><div class="MsoNormal">Interstellar dust absorbs light, particularly at blue wavelengths. This dust absorption can lead to erroneous luminosity-color determinations, so<span style="color: #000c7c; font-family: Times; font-size: 16pt;"> </span>a Cepheid from an external galaxy will appear both –fainter and more distant and redder and cooler – than it actually is. Systematic errors due reddening, if not corrected, will affect the distance scale.</div><div class="MsoNormal">The role of metallicity in the evolution of individual Cepheids and how it affects P-L relation and classic B-W method has been a matter of debate for several decades. Gieren et al. (1998, 2005) found a significantly different slope between the Galactic and LMC samples of Cepheids based on ISB analysis of Galactic and LMC Cepheids.</div><div class="MsoNormal">Trigonometric parallaxes for RR Lyrae were determined by HIPPARCOS and HST. However, only data for RR Lyr and 2 others were considered to be accurate enough (Benedict et al., 2002, Carretta and Gratton 2000).<span class="MsoFootnoteReference"> </span>This makes calibration for P-L and B-W methods difficult.</div><h2>References:</h2><div class="MsoNormal" style="margin-left: 14.2pt; text-indent: 0cm;">BENEDICT et al., 2002, ASTROMETRY<sup> </sup>WITH<sup> </sup>THE<sup> </sup><i>HUBBLE</i><i><sup> </sup>SPACE</i><i><sup> </sup>TELESCOPE</i>:<sup> </sup>A<sup> </sup>PARALLAX<sup> </sup>OF<sup> </sup>THE<sup> </sup>FUNDAMENTAL<sup> </sup>DISTANCE<sup> </sup>CALIBRATOR<sup> </sup>RR<sup> </sup>LYRAE, THE ASTRONOMICAL JOURNAL, 123:473-484<o:p></o:p></div><div class="MsoNormal" style="margin-left: 14.2pt; text-indent: 0cm;">Carretta and Gratton 2000, Distances, Ages, and Epoch of Formation of Globular Clusters, The Astrophysical Journal, 533:215-235<o:p></o:p></div><div class="MsoNormal" style="margin-left: 14.2pt; text-indent: 0cm;">Carretta, Gratton, Clementini, 2000, Mon. Not. R. Astron. Soc. 316, 721±728 (2000)<o:p></o:p></div><div class="MsoNormal" style="margin-left: 14.2pt; text-indent: 0cm;">CaltechCepheidsWeb- <a href="http://nedwww.ipac.caltech.edu/level5/Cepheids/"><span style="text-decoration: none;">http://nedwww.ipac.caltech.edu/level5/Cepheids/</span></a><o:p></o:p></div><div class="MsoNormal" style="margin-left: 14.2pt; text-indent: 0cm;">CaltechRRLyraeWeb <a href="http://nedwww.ipac.caltech.edu/level5/March03/Vandenberg/Vandenberg3_2.html">http://nedwww.ipac.caltech.edu/level5/March03/Vandenberg/Vandenberg3_2.html</a><o:p></o:p></div><div class="MsoNormal" style="margin-left: 14.2pt; text-indent: 0cm;">Cox, J.P. 1980, in <a href="http://adsabs.harvard.edu/cgi-bin/bib_query?1980tsp..book.....C"><span style="text-decoration: none;">Theory of Stellar Pulsation</span></a>, (Princeton University Press, Princeton)<o:p></o:p></div><div class="MsoNormal" style="margin-left: 14.2pt; text-indent: 0cm;">Gieren, W.P., Fouque, P., & Gomez, M.; 1998, AJ, 496, 17<o:p></o:p></div><div class="MsoNormal" style="margin-left: 14.2pt; text-indent: 0cm;">Gieren, W., Storm, J., Barnes, T.G., et al.; 2005, ApJ, 627, 224<o:p></o:p></div><div class="MsoNormal" style="margin-left: 14.2pt; text-indent: 0cm;">HipparcosESAWeb- <a href="http://www.esa.int/science/hipparcos">http://www.esa.int/science/hipparcos</a><o:p></o:p></div><div class="MsoNormal" style="margin-left: 14.2pt; text-indent: 0cm;">Marengo M., Karovska M., Sasselov D., 2004, AN ERROR ANALYSIS OF THE GEOMETRIC BAADE-WESSELINK METHOD, The Astrophysical Journal, 603:285–291<o:p></o:p></div><div class="MsoNormal" style="margin-left: 14.2pt; text-indent: 0cm;">OhioStateAstronomyWeb- <a href="http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit4/cosdist.html">http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit4/cosdist.html</a><o:p></o:p></div><div class="MsoNormal" style="margin-left: 14.2pt; text-indent: 0cm;">Perryman et al., 1998, The Hyades: distance, structure, dynamics, and age, arXiv:astro-ph/9707253<o:p></o:p></div><div class="MsoNormal" style="margin-left: 14.2pt; text-indent: 0cm;">Perryman et al., 1999, <a href="http://adsabs.harvard.edu/abs/1997A&A...323L..49P"><u style="text-underline: #0016E7;"><span style="text-decoration: none;">"The HIPPARCOS Catalogue"</span></u></a>. Astronomy and Astrophysics 323: L49–L52. Retrieved 2008-10-18.<o:p></o:p></div><div class="MsoNormal" style="margin-left: 14.2pt; text-indent: 0cm;">Popowski and Gould, 1998, <a href="http://arxiv.org/abs/astro-ph/9703140"><u style="text-underline: #0016E7;"><span style="text-decoration: none;">"Mathematics of Statistical Parallax and the Local Distance Scale"</span></u></a>. arXiv, Ohio State University. Retrieved 2008-10-20.<o:p></o:p></div><span style="font-family: Georgia; font-size: 12pt;">ScienceVaultWeb- </span><span style="font-family: Georgia; font-size: 12pt;"><a href="http://sciencevault.net/ibphysics/astrophysics/stellardistances.htm"><span style="font-size: 9pt;">http://sciencevault.net/ibphysics/astrophysics/stellardistances.htm</span></a></span> <br />
<div style="mso-element: footnote-list;"><br />
<hr align="left" size="1" width="33%" /><div id="ftn" style="mso-element: footnote;"><div class="MsoFootnoteText"><a href="http://www.blogger.com/post-create.g?blogID=9016726145636701998#_ftnref" name="_ftn1" style="mso-footnote-id: ftn;" title=""><span class="MsoFootnoteReference"><span style="vertical-align: baseline;"><span class="Apple-style-span" style="font-size: xx-small;">[1]</span></span></span></a><span class="Apple-style-span" style="font-size: xx-small;"> The term spectroscopic parallax is a misnomer, as it actually has nothing to do with trigonometric parallax. It is, however, a legitimate way to find distances to stars.</span></div></div></div><br />
<span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.jtitle=The+Astrophysical+Journal&rft_id=info%3Adoi%2F10.1086%2F308629&rfr_id=info%3Asid%2Fresearchblogging.org&rft.atitle=Distances%2C+Ages%2C+and+Epoch+of+Formation+of+Globular+Clusters&rft.issn=0004-637X&rft.date=2000&rft.volume=533&rft.issue=1&rft.spage=215&rft.epage=235&rft.artnum=http%3A%2F%2Fstacks.iop.org%2F0004-637X%2F533%2Fi%3D1%2Fa%3D215&rft.au=Carretta%2C+E.&rft.au=Gratton%2C+R.&rft.au=Clementini%2C+G.&rft.au=Fusi+Pecci%2C+F.&rfe_dat=bpr3.included=1;bpr3.tags=Astronomy%2Cphysics%2C+astrophysics%2C+Instrumentation+and+Methods+for+Astrophysics">Carretta, E., Gratton, R., Clementini, G., & Fusi Pecci, F. (2000). Distances, Ages, and Epoch of Formation of Globular Clusters <span style="font-style: italic;">The Astrophysical Journal, 533</span> (1), 215-235 DOI: <a rev="review" href="http://dx.doi.org/10.1086/308629">10.1086/308629</a></span>Olga V. Vovkhttp://www.blogger.com/profile/10788084870025950079noreply@blogger.com1