======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 28, No. 2 Mar 7, 2008 Recently submitted papers: -------------------------- 1) Linearly and Circularly Polarized Emission in Sagittarius A* (Huang(1 et al., ApJL) 2) IRS 13N: a new comoving group of sources at the Galactic Center (Muzic et al., A&A) 3) On a Source of Systematic Error, in Absolute Measurement of Galactocentric Distance from Solving for the Stellar Orbit Around Sgr A* (Nikiforov, Dynamics) ------------------------------------------------------------------------ Email : muduri@shao.ac.cn Title : Linearly and Circularly Polarized Emission in Sagittarius A* Author(s): Lei Huang(1,3), Siming Liu(4), Zhi-Qiang Shen(1,2), Mike J. Cai(5), Hui Li,(4) and Christopher L. Fryer(4) Institute: (1) Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, China (2) Joint Institute for Galaxy and Cosmology (JOINGC) of ShAO and USTC, Shanghai 200030, China (3) Graduate School of the Chinese Academy of Sciences, Beijing 100039, China (4) Los Alamos National Laboratory, Los Alamos, NM 87545 (5) Academia Sinica, Institute of Astronomy and Astrophysics, Taipei 106, Taiwan Paper : ApJL, accepted Feb 2008 EPrint : 0802.3561 Web : http://arxiv.org/pdf/0802.3561 Abstract: We perform general relativistic ray-tracing calculations of the transfer of polarized synchrotron radiation through the relativistic accretion flow in Sagittarius (Sgr) A*. Based on a two-temperature magneto-rotational-instability (MRI) induced accretion mode, the birefringence effects are treated self-consistently. By fitting the spectrum and polarization of Sgr A* from millimeter to near-infrared bands, we are able to not only constrain the basic parameters related to the MRI and the electron heating rate, but also limit the orientation of the accretion torus. These constraints lead to unique polarimetric images, which may be compared with future millimeter and sub-millimeter VLBI observations. In combination with general relativistic MHD simulations, the model has the potential to test the MRI with observations of Sgr A*. ------------------------------------------------------------------------ Email : muzic@ph1.uni-koeln.de Title : IRS 13N: a new comoving group of sources at the Galactic Center Author(s): K. Muzic^1,2, R. Schoedel^3, A. Eckart^1,2, L. Meyer^4, A. Zensus^2,1 Institute: (1) I. Physikalisches Institut, Universitaet zu Koeln, Zuelpicher Str. 77, 50937 Koeln, Germany, (2) Max-Planck-Institut fuer Radioastronomie, Auf dem Huegel 69, 53121 Bonn, Germany, (3) Instituto de Astrofisica de Andalucia, Camino Bajo de Huétor 50, 18008 Granada, Spain, (4) University of California, Division of Astronomy and Astrophysics, Los Angeles, CA 90095-4705, Paper : A&A, 2008, in press Abstract: The Galactic Center IRS 13E cluster is located 3.2'' from SgrA*. It is an extremely dense stellar association containing several Wolf-Rayet and O-type stars, at least four of which show a common velocity. Only half an arcsecond north from IRS 13E there is a complex of extremely red sources, so-called IRS 13N. Their nature is still unclear. Based on the analysis of their colors, there are two main possibilities: (1) dust embedded sources older than few Myr, or (2) extremely young objects with ages less than 1Myr. We present the first proper motion measurements of IRS 13N members, and additionally give proper motions of four of IRS 13E stars resolved in the L'-band. The L'-band (3.8 micron ) observations have been carried out using the NACO adaptive optics system at the ESO VLT. Proper motions have been obtained by linear fitting of the stellar positions extracted by StarFinder as a function of time, weighted by positional uncertainties. We show that six of seven resolved northern sources show a common proper motion, thus revealing a new comoving group of stars in the central half parsec of the Milky Way. The common proper motions of IRS 13E and IRS 13N clusters are significantly (>5 sigma ) different. We also performed a fitting of the positional data for those stars onto Keplerian orbits, assuming SgrA* as the center of the orbit. Our results favor the very young stars hypothesis. ------------------------------------------------------------------------ Email : nii@dyna.astro.spbu.ru Title : On a Source of Systematic Error, in Absolute Measurement of Galactocentric Distance from Solving for the Stellar Orbit Around Sgr A* Author(s): Igor' I. Nikiforov Institute: Sobolev Astronomical Institute, St. Petersburg State University, Universitetskij pr. 28, Staryj Peterhof, St. Petersburg 198504, Russia Paper : Dynamics of Galaxies, Proceedings of the International Conference held at Pulkovo Observatory, August 6-10, 2007, in press EPrint : 0803.0825 Abstract: Eisenhauer et al. (2003, 2005) derived absolute (geometrical) estimates of the distance to the center of the Galaxy, R_0, from the star S2 orbit around Sgr A* on the assumption that the intrinsic velocity of Sgr A* is negligible. This assumption produces the source of systematic error in R_0 value owing to a probable motion of Sgr A* relative to the accepted velocity reference system which is arbitrary to some extent. Eisenhauer et al. justify neglecting all three spatial velocity components of Sgr A* mainly by low limits of Sgr A*'s proper motion of 20-60 km/s. In this brief paper, a simple analysis in the context of the Keplerian dynamics was used to demonstrate that neglect of even low (perhaps, formal) radial velocity of Sgr A* leads to a substantial systematic error in R_0: the same limits of 20-60 km/s result in R_0 errors of 1.3-5.6%, i.e., (0.1-0.45)* (R_0/8) kpc, for current S2 velocities. Similar values for Sgr A*'s tangential motion can multiply this systematic error in the case of S2 orbit by factor 1.5-1.9 in the limiting cases. ------------------------------------------------------------------------ (Older versions of the Newsflash can be found at the gcnews web-page) ======================================================================== Edited by Sera Markoff, Loránt Sjouwerman, Joseph Lazio, Cornelia Lang, Rainer Schödel, Masaaki Sakano, Feng Yuan - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - For Abstract submission please follow the instructions which are at http://www.aoc.nrao.edu/~gcnews/home/submission.shtml ========================================================================