======================================================================== 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. 11 Apr 19, 2000 Recently submitted papers: -------------------------- 1) Hybrid Thermal-Nonthermal Synchrotron Emission from Hot Accretion Flows (Oezel et al., ApJ) 2) Tidal spin-up of stars in dense stellar cusps around massive black holes (Alexander & Kumar, ApJ) ------------------------------------------------------------------------ Email : fozel@cfa.harvard.edu Title : Hybrid Thermal-Nonthermal Synchrotron Emission from Hot Accretion Flows Author(s): Feryal Oezel(1,2), Dimitrios Psaltis(1), and Ramesh Narayan(1) Institute: (1) Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138;, fozel,dpsaltis,rnarayan@cfa.harvard.edu (2) Physics Department, Harvard University Paper : ApJ, in press EPrint : astro-ph/0004195 Abstract: We investigate the effect of a hybrid electron population, consisting of both thermal and non-thermal particles, on the synchrotron spectrum, image size, and image shape of a hot accretion flow onto a supermassive black hole. We find two universal features in the emitted synchrotron spectrum: (i) a prominent shoulder at low ( <~ 10^11 Hz) frequencies that is weakly dependent on the shape of the electron energy distribution, and (ii) an extended tail of emission at high (> 10^13 Hz) frequencies whose spectral slope depends on the slope of the power-law energy distribution of the electrons. In the low-frequency shoulder, the luminosity can be up to two orders of magnitude greater than with a purely thermal plasma even if only a small fraction (< 1%) of the steady-state electron energy is in the non-thermal electrons. We apply the hybrid model to the Galactic center source, Sgr A^*. The observed radio and IR spectra imply that at most 1% of the steady-state electron energy is present in a power-law tail in this source. This corresponds to no more than 10% of the electron energy injected into the non-thermal electrons and hence 90 % into the thermal electrons. We show that such a hybrid distribution can be sustained in the flow because thermalization via Coulomb collisions and synchrotron self-absorption are both inefficient. The presence of non-thermal electrons enlarges the size of the radio image at low frequencies and alters the frequency dependence of the brightness temperature. A purely thermal electron distributions produces a sharp-edged image while a hybrid distribution causes strong limb brightening. These effects can be seen up to frequencies 10^11 Hz and are accessible to radio interferometers. ------------------------------------------------------------------------ Email : tal@ias.edu Title : Tidal spin-up of stars in dense stellar cusps around massive black holes Author(s): Tal Alexander & Pawan Kumar Institute: Institute for Advanced Study, Olden Lane, Princeton, NJ 08540 Paper : ApJ, submitted EPrint : astro-ph/0004240 Abstract: We show that main-sequence stars in dense stellar cusps around massive black holes are likely to rotate at a significant fraction of the centrifugal breakup velocity due to spin-up by hyperbolic tidal encounters. We use realistic stellar structure models to calculate analytically the tidal spin-up in soft encounters, and extrapolate these results to close and penetrating collisions using smoothed particle hydrodynamics simulations. We find that the spin-up falls off only slowly with distance from the black hole because the increased tidal coupling in slower collisions at larger distances compensates for the decrease in the stellar density. We apply our results to the stars near the massive black hole in the Galactic Center. Over their lifetime, 1 M_o main sequence stars in the inner 0.3 pc of the Galactic Center are spun-up on average to 10% - 30% of the centrifugal breakup limit. Such rotation is 20 - 60 times higher than is usual for such stars and may affect their subsequent evolution and their observed properties. ------------------------------------------------------------------------ (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 ========================================================================