======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 16, No. 3 Aug 29, 2003 Recently submitted papers: -------------------------- 1) Interferometric Detection of Linear Polarization from Sagittarius A* at 230 GHz (1 et al., ApJ) 2) Really Cool Stars and the Star Formation History at the Galactic Center (Blum1 et al., ApJ) ------------------------------------------------------------------------ Email : gbower@astron.berkeley.edu Title : Interferometric Detection of Linear Polarization from Sagittarius A* at 230 GHz Author(s): Geoffrey C. Bower 1, Melvyn C.H. Wright 1, Heino Falcke 2, Donald C. Backer 1 Institute: (1) 1 (2) 2 Paper : ApJ, 588, 331 EPrint : astro-ph/0302227 Abstract: We measured the linear polarization of Sagittarius A* to be 7.2 +/- 0.6 % at 230 GHz using the BIMA array with a resolution of 3.6 * 0.9 arcsec. This confirms the previously reported detection with the JCMT 14-m antenna. Our high resolution observations demonstrate that the polarization does not arise from dust but from a synchrotron source associated with Sgr A*. We see no change in the polarization position angle and only a small change in the polarization fraction in four observations distributed over 60 days. We find a position angle 139 +/- 4 degrees that differs substantially from what was found in earlier JCMT observations at the same frequency. Polarized dust emission cannot account for this discrepancy leaving variability and observational error as the only explanations. The BIMA observations alone place an upper limit on the magnitude of the rotation measure of 2* 10^6 \rdm. These new observations when combined with the JCMT observations at 150, 375 and 400 GHz suggest RM=-4.3 +/- 0.1 * 10^5 \rdm. This RM may be caused by an external Faraday screen. Barring a special geometry or a high number of field reversals, this RM rules out accretion rates greater than 10^-7 \msuny. This measurement is inconsistent with high accretion rates necessary in standard advection dominated accretion flow and Bondi-Hoyle models for Sgr A*. It argues for low accretion rates as a major factor in the overall faintness of Sgr A*. ------------------------------------------------------------------------ Email : rblum@ctio.noao.edu Title : Really Cool Stars and the Star Formation History at the Galactic Center Author(s): R. D. Blum1,Solange V. Ramirez2, K. Sellgren3, K. Olsen1 Institute: (1) Cerro Tololo Interamerican Observatory, Casilla 603, La Serena, Chile, rblum@ctio.noao.edu (2) SIRTF Science Center, JPL/Caltech, Pasadena, CA 91125, USA,solange@astro.caltech.edu (3) Astronomy Department, The Ohio State University, 140 West 18th Ave, Columbus, OH 43210, USA ,sellgren@astronomy.ohio-state.edu Paper : ApJ, Nov 2003, in press EPrint : astro-ph/0307291 Abstract: We present lambda /\Delta lambda = 550 to 1200 near infrared H and K spectra for a magnitude limited sample of 79 asymptotic giant branch and cool supergiant stars in the central 5 pc (diameter) of the Galaxy. We use a set of similar spectra obtained for solar neighborhood stars with known \teff and \mbol that is in the same range as the Galactic center (GC) sample to derive \teff and \mbol for the GC sample. We then construct the Hertzsprung-Russell (HRD) diagram for the GC sample. Using an automated maximum likelihood routine, we derive a coarse star formation history of the GC. We find (1) roughly 75% of the stars formed in the central few pc are older than 5 Gyr; (2) the star formation rate (SFR) is variable over time, with a roughly 4 times higher star formation rate in the last 100 Myr compared to the average SFR; (3) our model can only match dynamical limits on the total mass of stars formed by limiting the IMF to masses above 0.7 M_o. This could be a signature of mass segregation or of the bias toward massive star formation from the unique star formation conditions in the GC; (4) blue supergiants account for 12 % of the total sample observed, and the ratio of red to blue supergiants is roughly 1.5; (5) models with isochrones with [Fe/H] = 0.0 over all ages fit the stars in our HRD better than models with lower [Fe/H] in the oldest age bins, consistent with the finding of Ramirez et al. (2000) that stars with ages between 10 Myr and 1 Gyr have solar [Fe/H]. ------------------------------------------------------------------------ (Older versions of the Newsflash can be found at the gcnews web-page) ======================================================================== Edited by Angela Cotera Sera Markoff Lorant Sjouwerman (cotera@asu.edu) (sera@space.mit.edu) (lsjouwerman@aoc.nrao.edu) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - For Abstract submission please follow the instructions which are at http://www.aoc.nrao.edu/~gcnews/home/submission.shtml ========================================================================