======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 25, No. 5 Oct 6, 2006 Recently submitted papers: -------------------------- 1) Iron and Nickel Line Diagnostics for the Galactic Center Diffuse Emission (Koyama et al., PASJ) 2) Hypervelocity Collisions of Binary Stars at the Galactic Centre (Ginsburg & Loeb, MNRAS) 3) Constraints on the Stellar Mass Function from Stellar Dynamics at the Galactic Center (Alexander & Armitage, ApJ) ------------------------------------------------------------------------ Email : hyodo@cr.scphys.kyoto-u.ac.jp Title : Iron and Nickel Line Diagnostics for the Galactic Center Diffuse Emission Author(s): Katsuji Koyama(1), Yoshiaki Hyodo(1), Tatsuya Inui(1), Hiroshi Nakajima(1), Hironori Matsumoto(1), Takeshi Go Tsuru(1), Tadayuki Takahashi(2), Yoshitomo Maeda(2), Noriko Yamazaki(2), Hiroshi Murakami(3), Shigeo Yamauchi(4), Yohko Tsuboi(5), Atsushi Senda(6), Jun Kataoka(7), Hiromitsu Takahashi(8), Stephen S Holt(9), Gregory V Brown(10) Institute: (1) Department of Physics, Graduate school of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502 (2) Institute of Space and Astronautical Science, JAXA, Sagamihara, Kanagawa, 229-8510 (3) PLAIN center, ISAS/JAXA, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510 (4) Faculty of Humanities and Social Sciences, Iwate University, 3-18-34 Ueda, Morioka, Iwate 020-8550 (5) Department of Physics, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga,, Bunkyo-ku, Tokyo 112-8551, Japan (6) The Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako, Saitama 351-0198 (7) Department of Physics, Tokyo Institute of Technology, Meguro, Tokyo, 152-8551 (8) Department of Physical Science, School of Science, Hiroshima University, 1-3-1 Kagamiyama,, Higashi-Hiroshima, Hiroshima 739-8526 (9) Olin College, Needham, MA 02492 USA (10) High Energy Density Physics and Astrophysics Division, Lawrence Livermore National Laboratory,, Livermore, California 94551, USA Paper : PASJ, accepted EPrint : astro-ph/0609215 Abstract: We have observed the diffuse X-ray emission from the Galactic center (GC) using the X-ray Imaging Spectrometer (XIS) on Suzaku. The high-energy resolution and the low-background orbit provide excellent spectra of the GC diffuse X-rays (GCDX). The XIS found many emission lines in the GCDX near the energy of K-shell transitions of iron and nickel. The most pronounced features are Fe I K alpha at 6.4 keV and K-shell absorption edge at 7.1 keV, which are from neutral and/or low ionization states of iron, and the K-shell lines at 6.7 keV and 6.9 keV from He-like (Fe XXV K alpha ) and hydrogenic (Fe XXVI Ly alpha ) ions of iron. In addition, K alpha lines from neutral or low ionization nickel (Ni I K alpha ) and He-like nickel (Ni XXVII K alpha ), and Fe I K beta , Fe XXV K beta , Fe XXVI Ly beta , Fe XXV K gamma and Fe XXVI Ly gamma are detected for the first time. The line center energies and widths of Fe XXV K alpha and Fe XXVI Ly alpha favor a collisional excitation (CE) plasma for the origin of the GCDX. The electron temperature determined from the line flux ratio of Fe XXV-K alpha / Fe XXV-K beta is similar to the ionization temperature determined from that of Fe XXV-K alpha /Fe XXVI-Ly alpha . Thus it would appear that the GCDX plasma is close to ionization equilibrium. The 6.7 keV flux and temperature distribution to the galactic longitude is smooth and monotonic, in contrast to the integrated point source flux distribution. These facts support the hypothesis that the GCDX is truly diffuse emission rather than the integration of the outputs of a large number of unresolved point sources. In addition, our results demonstrate that the chemical composition of Fe in the interstellar gas near the GC is constrained to be about 3.5 times solar. ------------------------------------------------------------------------ Email : iginsbur@cfa.harvard.edu Title : Hypervelocity Collisions of Binary Stars at the Galactic Centre Author(s): Idan Ginsburg, Abraham Loeb Institute: Harvard-Smithsonian Center for Astrophysics, 60 Garden St., MS 51, Cambridge, MA 02138, USA Paper : MNRAS, September 2006 EPrint : astro-ph/0609440 Abstract: Recent surveys have identified seven hypervelocity stars (HVSs) in the halo of the Milky Way. Most of these stars may have originated from the breakup of binary star systems by the nuclear black hole SgrA*. In some instances, the breakup of the binary may lead to a collision between its member stars. We examine the dynamical properties of these collisions by simulating thousands of different binary orbits around SgrA* with a direct N-body integration code. For some orbital parameters, the two stars collide with an impact velocity lower than their escape velocity and may therefore coalesce. Some of the massive S-stars near Sgr A* might be the merger remnants of binary systems. ------------------------------------------------------------------------ Email : rda@jilau1.colorado.edu Title : Constraints on the Stellar Mass Function from Stellar Dynamics at the Galactic Center Author(s): R.D. Alexander, M.C. Begelman & P.J. Armitage Institute: JILA, University of Colorado, Boulder, CO 80309-0440 Paper : ApJ, accepted Web : http://arxiv.org/abs/astro-ph/0609812 Abstract: We consider the dynamical evolution of a disk of stars orbiting a central black hole. In particular, we focus on the effect of the stellar mass function on the evolution of the disk, using both analytic arguments and numerical simulations. We apply our model to the ring of massive stars at 0.1pc from the Galactic Center, assuming that the stars formed in a cold, circular disk, and find that our model requires the presence of a significant population of massive (>100Msun) stars in order to explain the the observed eccentricities of 0.2-0.3. Moreover, in order to limit the damping of the heavier stars' eccentricities, we also require fewer low-mass stars than expected from a Salpeter mass function, giving strong evidence for a significantly ``top-heavy'' mass function in the rings of stars seen near to the Galactic Center. We also note that the maximum possible eccentricities attainable from circular initial conditions at ages of <10Myr are around 0.4-0.5, and suggest that any rings of stars found with higher eccentricities were probably not formed from circular disks. ------------------------------------------------------------------------ (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, Robin Herrnstein - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - For Abstract submission please follow the instructions which are at http://www.aoc.nrao.edu/~gcnews/home/submission.shtml ========================================================================