======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 27, No. 12 Dec 13, 2007 Recently submitted papers: -------------------------- 1) Constraints on jet-driven disk accretion in Sagittarius A* (Jolley & Kuncic, ApJ) 2) Dynamical and evolutionary constraints, on the nature and origin of hypervelocity stars (Perets, proceedings) 3) Suzaku X-Ray Spectroscopy of a Peculiar Hot Star in the Galactic Center Region (Hyodo et al., PASJ) 4) Massive Star Formation Near Sgr A* and Bimodal Star Formation in the Nuclear Disk (Zadeh & Wardle, proceedings) ------------------------------------------------------------------------ Email : erin@physics.usyd.edu.au Title : Constraints on jet-driven disk accretion in Sagittarius A* Author(s): Erin J. D. Jolley and Zdenka Kuncic Institute: (1) School of Physics, University of Sydney, Sydney NSW, Australia Paper : ApJ, March 2008, in press EPrint : 0711.4626 Abstract: We revisit theoretical and observational constraints on geometrically-thin disk accretion in Sagittarius A* (Sgr A*). We show that the combined effects of mass outflows and electron energization in the hot part of the accretion flow can deflate the inflowing gas from a geometrically-thick structure. This allows the gas to cool and even thermalize on an inflow timescale. As a result, a compact, relatively cool disk may form at small radii. We show that magnetic coupling between the relativistic disk and a steady-state jet results in a disk that is less luminous than a standard relativistic disk accreting at the same rate. This relaxes the observational constraints on thin-disk accretion in Sgr A* (and by implication, other Low-Luminosity Active Galactic Nulcei, LLAGN). We find typical cold gas accretion rates of a few * 10^-9 M_o yr^-1. We also find that the predicted modified disk emission is compatible with existing near-infrared (NIR) observations of Sgr A* in its quiescent state provided that the disk inclination angle is >approx 87^o and that the jet extracts more than 75% of the accretion power. ------------------------------------------------------------------------ Email : hagai.perets@weizmann.ac.il Title : Dynamical and evolutionary constraints, on the nature and origin of hypervelocity stars Author(s): Hagai B. Perets EPrint : 0712.1888 Abstract: In recent years several hypervelocity stars (HVSs) have been observed in the halo of our Galaxy. Such stars are thought to be ejected through dynamical interactions near the massive black hole (MBH) in the Galactic center. Three scenarios have been suggested for their ejection; binary disruption by a MBH, scattering by inspiraling IMBH and scattering by stellar BHs close to MBH. These scenarios involve different stellar populations in the Galactic center. Here we use observations of the Galactic center stellar population, dynamical and evolutionary arguments to constrain the nature and origin of HVSs. We show that the IMBH inspiral scenario requires too many ( 10^3) main sequence B stars to exist close to the MBH (<0.01 pc) at the time of inspiral, where current observations show ( 10) such stars. Scattering by SBHs are also not likely to be consistent with the observed population of B stars in the Galactic center, although this scenario can still be compatible with observations under extreme conditions. The binary disruption scenario is still consistent with current observations. In addition we show that due to the conditions close to the MBH most binary star systems are not expected to survive for long in this region. Consequently, unique stellar populations that require long evolution in binaries are not expected to be ejected as HVSs in the BHs scattering mechanisms (this may also be related to to the recently observed asymmetry in the velocity distribution of HVSs). Furthermore, it is shown that recently suggested signatures for HVSs origin such as hypervelocity binaries and slow rotating HVSs may be much weaker than suggested and require large statistics. ------------------------------------------------------------------------ Email : hyodo@cr.scphys.kyoto-u.ac.jp Title : Suzaku X-Ray Spectroscopy of a Peculiar Hot Star in the Galactic Center Region Author(s): Yoshiaki Hyodo(1) Masahiro Tsujimoto(2,3) Katsuji Koyama(1) Shogo Nishiyama(4) Tetsuya Nagata(5) Itsuki Sakon(6) Hiroshi Murakami(7) and Hironori Matsumoto(1) Institute: (1) Department of Physics, Graduate School of Science, Kyoto University,,Kita-shirakawa Oiwake-cho, Sakyo, Kyoto 606-8502 (2) Department of Astronomy \& Astrophysics, Pennsylvania State University, 525 Davey Laboratory, University Park, PA 16802, USA (3) Department of Physics, Rikkyo University, 3-34-1, Nishi-Ikebukuro, Toshima, Tokyo 171-8501 (4) National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, Tokyo 181-8588 (5) Department of Astronomy, Graduate School of Science, Kyoto University,,Kita-shirakawa Oiwake-cho, Sakyo, Kyoto 606-8502 (6) Department of Astronomy, School of Science, the University of Tokyo, 7-3-1, Hongo, Bunkyo, Tokyo 113-0033 (7) Institute of Space and Astronautical Science, 3-1-1, Yoshinodai, Sagamihara, Kanagawa, 229-8510 Paper : PASJ, 60, SP1, 2008 in press EPrint : 0712.0280 Abstract: We present the results of a Suzaku study of a bright point-like source in the 6.7 keV intensity map of the Galactic center region. We detected an intense Fe XXV 6.7 keV line with an equivalent width of 1 keV as well as emission lines of highly ionized Ar and Ca from a spectrum obtained by the X-ray Imaging Spectrometer. The overall spectrum is described very well by a heavily absorbed ( 2*10^23 cm^-2) thin thermal plasma model with a temperature of 3.8 +/- 0.6 keV and a luminosity of 3*10^34 erg s^-1 (2.0-8.0 keV) at 8 kpc. The absorption, temperature, luminosity, and the 6.7 keV line intensity were confirmed with the archived XMM-Newton data. The source has a very red (J-K_s=8.2 mag) infrared spectral energy distribution (SED), which was fitted by a blackbody emission of 1000 K attenuated by a visual extinction of 31 mag. The high plasma temperature and the large X-ray luminosity are consistent with a wind-wind colliding Wolf-Rayet binary. The similarity of the SED to those of the eponymous Quintuplet cluster members suggests that the source is a WC-type source. ------------------------------------------------------------------------ Email : zadeh@northwestern.edu Title : Massive Star Formation Near Sgr A* and Bimodal Star Formation in the Nuclear Disk Author(s): F. Yusef-Zadeh(1) \& M. Wardle(2) Institute: (1) Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208 (2) Department of Physics, Macquarie University, Sydney NSW 2109 EPrint : 0712.1245 Abstract: The history of star formation in the strong gravitational potential of the Galactic center has been of much interest, recently. We propose that the sub-parsec-scale disk of massive stars orbiting the massive black hole at the Galactic center can be interpreted in terms of partial accretion of extended Galactic center clouds, such as the 50 km/s molecular cloud, as these clouds envelop Sgr A* on their passage through the inner Galactic center. The loss of angular momentum of the captured cloud material by self-interaction subsequent to gravitationally focusing by Sgr A* naturally creates a compact gaseous disk of material close to Sgr A* in which star formation takes place. On a larger scale the formation of massive clusters such as the Arches and Quintuplet clusters or on-going massive star formation such as Sgr B2 could also be triggered by cloud-cloud collisions due to gravitational focusing in the deep potential of the central bulge. Unlike the violent and high-pressure environment of clustered star formation triggered by cloud-cloud collision, there are also isolated pockets of star formation and quiescent dense clouds. These sites suggest an inefficient, slow mode of star formation. We propose enhanced cosmic rays in the nuclear disk may be responsible for inhibiting the process of star formation in this region. In particular, we argue that the enhanced ionization rate due to the impact of cosmic-ray particles is responsible for lowering the efficiency of on-going star formation in the nuclear disk of our Galaxy. The higher ionization fraction and higher thermal energy due to the impact of these electrons may also reduce MHD wave damping which contributes to the persistence of the high velocity dispersion of the molecular gas in the nuclear disk. ------------------------------------------------------------------------ (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 ========================================================================