======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@mpifr-bonn.mpg.de http://www.mpifr-bonn.mpg.de/gcnews ======================================================================== Vol. 11, No. 6 Mar 21, 2000 Recently submitted papers: -------------------------- 1) The Distance to the Galactic Center (MCNAMARA et al., PASP) 2) A Cluster of Black Holes at the Galactic Center (Miralda-Escude & Gould, ApJ) 3) Studies of Galactic Nuclei with Darwin (Wilson, ESA Proc.) ------------------------------------------------------------------------ Email : PASP@io.byu.edu Title : The Distance to the Galactic Center Author(s): D. H. MCNAMARA, J. B. MADSEN, J. BARNES, AND B. F. ERICKSEN Institute: (1) Department of Physics and Astronomy, Brigham Young University, Provo, UT 84602 Paper : PASP 112, 202-216, 2000 February Weblink : http://www.journals.uchicago.edu/PASP/journal/issues/v112n768/200037/200037.html Abstract: The Optical Gravitational Lensing Experiment (OGLE) data on high-amplitude delta Scuti stars (HADS) and RR Lyrae stars have been analyzed to determine the distance to the Galactic bulge. Individual color excesses and V absorption for each individual variable star have been computed, which lead to accurate V0 magnitudes. The period-luminosity relation of the HADS is utilized to calculate distance moduli. We find that the metal-strong HADS are concentrated strongly to the bulge. The best distance modulus to the bulge from the HADS data is 14.49 ± 0.06 mag. The dependence of Mv on period for the bulge RR Lyrae stars can be found from the reddening-free magnitude VV-I. The zero point of the Mvlog P relation is set with the horizontal branches of Oosterhoff type I globular clusters. Mv 0.44 mag (long-distance scale) in the log P interval -0.29 to -0.22 as inferred by main-sequence fitting of the clusters to local subdwarfs and utilizing short-period HADS in the clusters. The best distance modulus to the bulge from the RR Lyrae (ab) stars is 14.45 mag, which compares favorably with the distance modulus of 14.49 mag found from the HADS. We adopt the mean value 14.47 ± 0.08 mag for the bulge distance modulus. A small additive correction leads to 7.9 ± 0.3 kpc as the best distance to the Galactic center. ------------------------------------------------------------------------ Email : jordi@astronomy.ohio-state.edu Title : A Cluster of Black Holes at the Galactic Center Author(s): Jordi Miralda-Escude and Andrew Gould Institute: Department of Astronomy, The Ohio State University, Columbus, OH 43210, USA Paper : ApJ, submitted EPrint : astro-ph/0003269 Abstract: If the stellar population of the bulge contains black holes formed in the final core collapse of ordinary stars with M > 30 M_o, then about 25,000 stellar mass black holes should have migrated by dynamical friction into the central parsec of the Milky Way, forming a black hole cluster around the central supermassive black hole. These black holes can be captured by the central black hole when they randomly reach a highly eccentric orbit due to relaxation, either by direct capture (when their Newtonian peribothron is less than 4 Schwarzschild radii), or after losing orbital energy through gravitational waves. The overall depletion timescale is 30 Gyr, so most of the 25,000 black holes remain in the central cluster today. The presence of this black hole cluster would have several observable consequences. First, the low-mass, old stellar population should have been expelled from the region occupied by the black hole cluster due to relaxation, implying a core in the profile of solar-mass red giants with a radius of 2 \pc (i.e., 1'). The observed central density cusp (which has a core radius of only a few arc seconds) should be composed primarily of young (< 1 Gyr) stars. Second, flares from stars being captured by supermassive black holes in other galaxies should be rarer than usually expected because the older stars will have been expelled from the central regions by the black hole clusters of those galaxies. Third, the young (< 2 Gyr) stars found at distances 3 - 10 \pc from the Galactic center should be preferentially on highly eccentric orbits. Fourth, if future high-resolution K-band images reveal sources microlensed by the Milky Way's central black hole, then the cluster black holes could give rise to secondary (``planet-like'') perturbations on the main event. ------------------------------------------------------------------------ Email : wilson@astro.umd.edu Title : Studies of Galactic Nuclei with Darwin Author(s): Andrew S. Wilson Institute: Astronomy Department, University of Maryland, College Park, MD 20742, U.S.A., and Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 Paper : in ``Darwin and Astronomy: The Infrared Space Interferometer'', European Space Agency Publication Division, ESA SP-451 (2000) Abstract: This paper describes a number of potential observing programs for Darwin in the area of galactic nuclei. Observations of our Galactic centre at the shorter wavelengths (5 micron ) covered by Darwin will detect large numbers of stars. Extrapolation of the existing K band counts to the flux levels expected to be reached by Darwin in 1 hour suggests 5,000 detectable stars (arc sec)^-2 around the compact radio source Sgr A^*, which is commonly believed to be associated with a black hole of mass 2.6 million M_o. Over its lifetime, Darwin will be capable of exquisitely accurate measurements of stellar proper motions, including the ability to follow entire stellar orbits around the hole for stars with orbital radii 10^-3 pc. It may also be able to observe the general relativistic advance of the pericenter of stellar orbits somewhat closer to the hole. Stars behind the hole will be gravitationally lensed by it. It appears that the probability of observing a lensing event at any given time is of order unity at Darwin's resolution and sensitivity, so this should be a rich area of research. Darwin will measure the infrared spectrum of Sgr A^* with a gain of 4 orders of magnitude of sensitivity over present upper limits, easily detecting the source if it conforms to the predictions of current ADAF models. Stars which approach too close to the black hole will be tidally disrupted by it, and Darwin's resolution is sufficiently good to spatially separate such a tidally disrupted red giant or supergiant from the black hole itself. However, the event rate is almost certainly too low for such a disruption to be seen in the mission lifetime. Dust shells around stars in the vicinity of the Galactic centre will be promising candidates for study. Darwin will be able to study the stellar luminosity function and stellar proper motions in the nucleus of M31. The possibility of using the stellar proper motions to estimate the mass of the nuclear black hole is discussed. Potential investigations of the distributions of warm dust and gas in the nuclei of nearby active galaxies are described. Darwin should be able to image the dusty accretion disks in Seyfert galaxies, compact starbursts around active nuclei and, depending on the available spectral resolution, study the stratification of different ionic species through observations of fine structure lines. It may be possible to resolve the broad line region if the wavelength coverage extends to short enough wavelengths. Darwin's nulling mode will be valuable for blocking the bright, compact light of quasars and investigating the kinematics of the surrounding gas. Whether stars, gas or dust are observed, galactic nuclei present densely-populated, bright fields, so the primary technical requirement for good imaging fidelity is very extensive coverage of the uv plane. ------------------------------------------------------------------------ (Older versions of the Newsflash can be found at the gcnews web-page) ======================================================================== Edited by Angela Cotera Heino Falcke (cotera@ipac.caltech.edu) (hfalcke@mpifr-bonn.mpg.de) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - For Abstract submission please send the (La)Tex file of your paper to gcnews@mpifr-bonn.mpg.de ========================================================================