Subject: GCFLASH - Vol. 6, No. 2 (Aug 13, 1997) ======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@astro.umd.edu http://www.astro.umd.edu/~gcnews ======================================================================== Vol. 6, No. 2 Aug 13, 1997 Both editors of GCNEWS will be at the IAU next week, so please do not expect any email response during that period. Recently submitted papers: -------------------------- 1) Polarized Far-infrared Emission from the Core and Envelope of the Sagittarius B2 Molecular Cloud (Novak et al., ApJ) 2) Hydrodynamical Accretion Onto Sgr A* from Distributed Point Sources (Coker & Melia, ApJ) 3) On The Nature Of The EGRET Source at The Galactic Center (Sera et al., ApJ) ------------------------------------------------------------------------ Email : novak@clark.phys.nwu.edu Title : Polarized Far-infrared Emission from the Core and Envelope of the Sagittarius B2 Molecular Cloud Author(s): G. Novak(1), J.L. Dotson(1), C.D. Dowell(2), R.H. Hildebrand(2), S.R. Platt(2), D. A. Schleuning(2), P.F. Goldsmith(3) Institute: (1) Department of Physics and Astronomy, Northwestern University (2) Department of Astronomy and Astrophysics, and Enrico Fermi Institute, University of Chicago (3) National Astronomy and Ionosphere Center and Department of Astronomy, Cornell University Paper : ApJ, in press Abstract: We have detected linear polarization in the 115 micron continuum radiation from the giant molecular cloud Sagittarius B2. We found polarization at nine positions in the dense cloud core, and at fifteen positions in the less-dense envelope. The polarization mechanism in the core is polarization due to absorption by magnetically aligned grains, and that in the envelope is polarized emission from magnetically aligned grains. The inferred magnetic field direction is roughly North-South everywhere, but with spatially smooth variations of up to 30 degrees. By considering our data together with Zeeman splitting observations we are able to set a conservative lower limit of 150 microGauss on the strength of the large-scale field in the envelope. If large-scale fields this strong are common in Galactic Center clouds, they could be detectable via large-beam Zeeman measurements. For positions in the envelope that are furthest from the core, the field is nearly parallel to the plane of the Galaxy. This is consistent with the idea of a globally azimuthal magnetic field in the Galactic Center neutral gas layer, which is expected if gravitational forces dominate magnetic forces. ------------------------------------------------------------------------ Email : rfc@physics.arizona.edu Title : Hydrodynamical Accretion Onto Sgr A* from Distributed Point Sources Author(s): Robert F. Coker(1,3) and Fulvio Melia(1,2,4) Institute: (1) Physics Department, The University of Arizona, Tucson, AZ 85721 (2) Steward Observatory, The University of Arizona, Tucson, AZ 85721 (3) NASA GSRP Fellow (4) Presidential Young Investigator. Paper : accepted by ApJL Weblink : http://www.physics.arizona.edu/~rfc/ptsrc.ps.Z EPrint : astro-ph/9708089 Abstract: Spectral and kinematic studies suggest that the nonthermal radio source Sgr A*, located at the center of the Milky Way, is a supermassive compact object with a mass ~ 2-3*10^6 M_sun. Winds from nearby stars, located ~ 0.06 pc to the east of Sgr A*, should, in the absence of any outflow from the putative black hole itself, be accreting onto this object. We report the results of the first 3D Bondi-Hoyle hydrodynamical numerical simulations of this process under the assumption that the Galactic center wind is generated by several different point sources (here assumed to be 10 pseudo-randomly placed stars). Our results show that the accretion rate onto the central object can be higher than in the case of a uniform flow since wind-wind shocks dissipate some of the bulk kinetic energy and lead to a higher capture rate for the gas. However, even for this highly non-uniform medium, most of the accreting gas carries with it a relatively low level of specific angular momentum, though large transient fluctuations can occur. Additionally, the post-bow-shock focusing of the gas can be substantially different than that for a uniform flow, but it depends strongly on the stellar spatial distribution. We discuss how this affects the morphology of the gas in the inner 0.15 pc of the Galaxy and the consequences for accretion disk models of Sgr A*. ------------------------------------------------------------------------ Email : sera@gamma.physics.arizona.edu Title : On The Nature Of The EGRET Source at The Galactic Center Author(s): Sera Markoff(1,3), Fulvio Melia(1,2,4), and Ina Sarcevic Institute: (1) Physics Department, The University of Arizona, Tucson, AZ 85721 (2) Steward Observatory, The University of Arizona, Tucson, AZ 85721 (3) NSF Graduate Fellow. (4) Presidential Young Investigator. Paper : ApJ Letters, in press Weblink : http://www.physics.arizona.edu/~sera/apjlett.ps EPrint : astro-ph/9708068 Abstract: The recent detection of a gamma-ray flux from the direction of the Galactic center by EGRET on the Compton GRO raises the question of whether this is a point source (possibly coincident with the massive black hole candidate Sgr A*) or a diffuse emitter. Using the latest experimental particle physics data and theoretical models, we examine in detail the gamma-ray spectrum produced by synchrotron, inverse Compton scattering and mesonic decay resulting from the interaction of relativistic protons with hydrogen accreting onto a point-like object. Such a distribution of high-energy baryons may be expected to form within an accretion shock as the inflowing gas becomes supersonic. This scenario is motivated by hydrodynamic studies of Bondi-Hoyle accretion onto Sgr A*, which indicate that many of its radiative characteristics may ultimately be associated with energy liberated as this plasma descends down into the deep potential well. Earlier attempts at analyzing this process concluded that the EGRET data are inconsistent with a massive point-like object. Here, we demonstrate that a more careful treatment of the physics of p-p scattering suggests that a ~ 10^6 M_o black hole may be contributing to this high-energy emission. ------------------------------------------------------------------------ (Older versions of the Newsflash can be found at the gcnews web-page) ======================================================================== Edited by Angela Cotera Heino Falcke (cotera@ipac.caltech.edu) (hfalcke@astro.umd.edu) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - For Abstract submission please send the (La)Tex file of your paper to gcnews@astro.umd.edu ========================================================================