======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 24, No. 4 Apr 27, 2006 An announcement from Patricia Whitelock, President IAU Division VII: "NEW RESULTS ON THE GALACTIC CENTRE" A scientific meeting of Division VII (Galactic Systems) to be held at the Prague General Assembly Monday 21 August This is a full day meeting at the General Assembly which will include the business session of Division VII, but the emphasis will be on the science, and this year on the Galactic Centre. The programme will include about 8 half-hour presentations covering broad topics relating to SgrA* and star formation in the surrounding environment. The Centre of our Galaxy will also be contrasted with those of similar galaxies, and plenty of time will be allowed for discussion. One of the primary objectives of the meeting is to gauge the community interest in setting up an IAU Working Group on the Galactic Centre. This will be discussed and if appropriate an organizing committee nominated at the end of the science session. We expect to have mostly invited reviews, but there may be time for a few brief contributed talks. There is space for posters and those interested in contributing a poster should send the following information to me (paw@saao.ac.za) before May 15: 1. Authors names and affiliations (please be clear which author will present the poster at the meeting) 2. Title of the poster 3. Abstract of no more than 150 words. Contributed oral presentations will be selected from among the posters. SOC for meeting: Cornelia Lang (University of Iowa, USA) Joseph Lazio (Naval Research Laboratory, USA) Reinhard Genzel (MPI Extraterrestrial Physics, Germany) Ortwin Gerhard (MPI Extraterrestrial Physics, Germany) Patricia Whitelock (SA Astronomical Observatory & Univ Cape Town, South Africa) Invited Speakers include: Marcella Carollo, Andrea Ghez, Sera Markoff, Thibaut Paumard, Loic Rolland, Simon Portegies Zwart More detailed information will shortly be available via the web page of IAU commissions 33, 37 and Division VII. Patricia Whitelock (President IAU Division VII) ======================================================================== Recently submitted papers: -------------------------- 1) The pattern of accretion flow onto Sgr A* (Mo\'scibrodzka et al., astro-ph/0604516) 2) A Viscous Heating Mechanism for the Hot Plasma in the Galactic Center Region (Tagger, A&A) 3) Relativistic periastron precession and frame dragging of stellar orbits in the central arcsecond of our Galaxy (Kraniotis, Classical) 4) Hot Accretion With Conduction: Spontaneous Thermal Outflows (Tanaka & Menou, ApJ) 5) Recent star formation in the inner Galactic Bulge seen by ISOGAL (Schuller et al., A&A) ------------------------------------------------------------------------ Email : mmosc@camk.edu.pl Title : The pattern of accretion flow onto Sgr A* Author(s): Monika Mo\'scibrodzka^1, Tapas K. Das^2, Bozena Czerny^1 Institute: (1) N. Copernicus Astronomical Center, Bartycka 18, 00-716 Warsaw, Poland (2) Harish Chandra Research Institute, Allahabad 211 019, India Paper : astro-ph/0604516 EPrint : astro-ph/0604516 Abstract: The material accreting onto Sgr A* most probably comes from the nearby stars. We analyze the pattern of this flow at distances of a fraction of a parsec and we argue that the net angular momentum of this material is low but non-negligible, and the initially supersonic disk accretion changes into subsonic flow with constant angular momentum. Next we estimate the flow parameters at a distance R_BHL from the black hole and we argue that for the plausible parameter range the accretion flow is non-stationary. The inflow becomes supersonic at distance of 10^4 R_g but the solution does not continue below the horizon and the material piles up forming a torus, or a ring, at a distance of a few up to tens of Schwarzchild radii. Such a torus is known to be unstable and may explain strong variability of the flow in Sgr A*. Our considerations show that the temporary formation of such a torus seems to be unavoidable. Our best fitting model predicts a rather large accretion rate of around 4 * 10^-6 M_o/ yr directly on Sgr A*. We argue that magnetic fields in the flow are tangled and this allows our model to overcome the disagreement with the Faraday rotation limits. ------------------------------------------------------------------------ Email : belmont@astro.UCLA.EDU Title : A Viscous Heating Mechanism for the Hot Plasma in the Galactic Center Region Author(s): R. Belmont and M. Tagger Institute: (1) CEA Service d'Astrophysique, UMR "AstroParticlues et Cosmologie", Orme de merisiers, 91191 Gif-sur-Yvette, France Paper : A&A, Mar 2006, in press EPrint : astro-ph/0603499 Abstract: In addition to lines originating in a soft phase at 0.8 keV and to cold molecular clouds, the X-ray spectra from the Galactic center region also exhibit properties similar to those of a diffuse, thin, very hot plasma at 8 keV on a scale of hundreds of parsecs. This phase is surprising for more than one reason. First, such a hot plasma should not be bound to the Galactic plane and the power needed to sustain the escaping matter would be higher then any known source. Second, there is no known mechanism able to heat the plasma to more than a few keV. Recently we have suggested that, hydrogen having escaped, the hot plasma could be a helium plasma, heavy enough to be gravitationally confined. In this case, the required power is much more reasonable. We present here a possible heating mechanism which taps the gravitational energy of the molecular clouds. We note that the 8 keV plasma is highly viscous and we show how viscous friction of molecular clouds flowing within the hot phase can dissipate energy in the gas and heat it. We detail the MHD wake of a spherical cloud by considering the different MHD waves the cloud can excite. We find that most of the energy is dissipated by the damping of Alfvenic perturbations in two possible manners, namely by non-linear effects and by a large scale curvature of the field lines. We find that the total dissipation rate depends on the field strength. For fields B <=sssim 200 mu G both mechanisms produce power comparable to or higher than the radiative losses; for strong fields B > 1 mG, only the curvature damping can balance the X-ray emission and requires a radius of curvature R_c<= 100 pc; whereas for intermediate fields, the total dissipation is more than one order of magnitude smaller, requiring a higher accretion rate. We note that the plasma parameters may be optimal to make the dissipation most efficient, suggesting a self-regulation mechanism. The loss of kinetic and gravitational energy also causes accretion of the clouds and may have significant action on the gas dynamics in this region between the large scale, bar dominated flow and the central accretion to the massive black hole. ------------------------------------------------------------------------ Email : kraniotis@physics.tamu.edu Title : Relativistic periastron precession and frame dragging of stellar orbits in the central arcsecond of our Galaxy Author(s): G. V. Kraniotis Institute: Institute for Fundamental Physics Texas A \& M University, College Station, TX 77843, USA Paper : Classical & Quantum Gravity Abstract: The geodesic equations of general relativity that describe motion of a test particle in Kerr spacetime are solved exactly including the contribution from the cosmological constant. By applying the exact solution for the precession of the point of closest approach for the orbit of the test particle around the Kerr field, we calculate the relativistic effect of periapsis advance for the observed orbits of S-stars in the central arcsecond of our galaxy, assuming that the galactic centre is a Kerr black hole, for various values of the Kerr parameter including those supported by recent observations. The observation of the predicted effects can provide an important test of the theory of general relativity at the strong field regime. In addition, we derive the exact solution of timelike non-spherical polar and non-polar orbits. Exact expressions for the periastron (periapsis) advance and frame dragging (Lense-Thirring) effect for a test particle in a polar, non-spherical orbit in the Kerr gravitational field are derived and applied for practical calculations around the galactic centre. We subsequently derive an analytical expression for the periapsis precession for the equatorial non-circular orbit of a test particle around a rotating mass whose surrounding curved spacetime geometry is described by the Kerr-de Sitter field. ------------------------------------------------------------------------ Email : kristen@astro.columbia.edu Title : Hot Accretion With Conduction: Spontaneous Thermal Outflows Author(s): Takamitsu Tanaka, Kristen Menou Institute: Department of Astronomy, Columbia University, 550 West 120th Street, New York, NY 10027 Paper : ApJ, Apr 2006, submitted Web : http://www.arxiv.org/abs/astro-ph/0604509 Abstract: Motivated by the low-collisionality of gas accreted onto black holes in Sgr A* and other nearby galactic nuclei, we study a family of 2D advective accretion solutions with thermal conduction. While we only impose global inflow, the accretion flow spontaneously develops bipolar outflows. The role of conduction is key in providing the extra degree of freedom (latitudinal energy transport) necessary to launch these rotating thermal outflows. The sign of the Bernoulli constant does not discriminate between inflowing and outflowing regions. Our parameter survey covers mass outflow rates from 0 to 13% of the net inflow rate, outflow velocities from 0 to 11% of the local Keplerian velocity and outflow opening angles from 0 to 60 degs. As the magnitude of conduction is increased, outflows can adopt a conical geometry, pure inflow solutions emerge, and the limit of 2D non-rotating Bondi-like solutions is eventually reached. These results confirm that radiatively-inefficient, hot accretion flows have a hydrodynamical propensity to generate bipolar thermal outflows. ------------------------------------------------------------------------ Email : schuller@mpifr-bonn.mpg.de Title : Recent star formation in the inner Galactic Bulge seen by ISOGAL Author(s): F. Schuller(1,2), A. Omont(1), I. S. Glass(3), M. Schultheis(1,4), M. P. Egan(5) and S. D. Price(6) Institute: (1) Institut d'Astrophysique de Paris, CNRS and Paris-VI University, 98 bis, Bd Arago, F-75014 Paris, France (2) Max Planck Institut fuer Radioastronomie, Auf dem Huegel 69, DE-53121 Bonn, Germany (3) South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa (4) Observatoire de Besanccon, CNRS, 41 bis, av. de l'Observatoire, B.P. 1615, F-25010 Besan\ccon Cedex, France (5) Air Force Research Laboratory, ODASD, 1851 s. Bell St., CM3, Suite 7000, Arlington, VA 22202, USA (6) Air Force Research Laboratory, Space Vehicles Directorate, 29 Randolph Road, Hanscom AFB, MA 01731, USA Paper : A&A, accepted EPrint : astro-ph/0603576 Abstract: The stellar populations in the central region of the Galaxy are poorly known because of the high visual extinction and very great source density in this direction. To use recent infrared surveys for studying the dusty stellar objects in this region. We analyse the content of a 20*20 arcmin^2 field centred at (l,b)=(-0.27d,-0.06d) observed at 7 and 15 micron as part of the ISOGAL survey. These ISO observations are more than an order of magnitude better in sensitivity and spatial resolution than the IRAS observations. The sources are cross-associated with other catalogues to identify various types of objects. We then derive criteria to distinguish young objects from post-main sequence stars. We find that a sample of about 50 young stellar objects and ultra-compact HII regions emerges, out of a population of evolved AGB stars. We demonstrate that the sources colours and spatial extents, as they appear in the ISOGAL catalogue, possibly complemented with MSX photometry at 21 micron, can be used to determine whether the ISOGAL sources brighter than 300 mJy at 15 micron (or [15]<4.5 mag) are young objects or late-type evolved stars. ------------------------------------------------------------------------ (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 ========================================================================