======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 23, No. 4 Jan 26, 2006 Recently submitted papers: -------------------------- 1) Clockwise Stellar Disk and the Dark Mass in the Galactic Center (Beloborodov1 et al., ApJ) 2) The Fate of Former Companions to Hypervelocity Stars Originating at the Galactic Center (Loeb, MNRAS) 3) Warm Water Vapor around Sagittarius B2 (Cernicharo et al., ApJ) 4) Dark Matter and the CACTUS Gamma-Ray Excess from Draco (Kamionkowski, astro-ph/0601249) 5) Resonant relaxation near a massive black hole: the stellar distribution and gravitational wave sources (Alexander, ApJ) ------------------------------------------------------------------------ Email : amb@phys.columbia.edu Title : Clockwise Stellar Disk and the Dark Mass in the Galactic Center Author(s): Andrei M. Beloborodov1,2, Yuri Levin3, Frank Eisenhauer4, Reinhard Genzel4,5, Thibaut Paumard4, Stefan Gillessen4, Thomas Ott4 Institute: (1) Physics Department and Columbia Astrophysics Laboratory, Columbia University, 538 West 120th Street New York, NY 10027 (2) Astro-Space Center of Lebedev Physical Institute, Profsojuznaja 84/32, Moscow 117810, Russia (3) Leiden Observatory, PO Box 9513, NL-2300 RA, Leiden, The Netherlands (4) Max-Planck Institut fuer extraterrestrische Physik (MPE), Garching, Germany (5) Department of Physics, University of California, Berkeley, CA 94720 Paper : ApJ, submitted EPrint : astro-ph/0601273 Abstract: Two disks of young stars have recently been discovered in the Galactic Center. The disks are rotating in the gravitational field of the central black hole at radii r 0.1-0.3 pc and thus open a new opportunity to measure the central mass. We find that the observed motion of stars in the clockwise disk implies M=(4.3+/- 0.5)* 10^6M_o for the fiducial distance to the Galactic Center R_0=8 kpc and derive the scaling of M with R_0. As a tool for our estimate we use orbital roulette, a recently developed method. The method reconstructs the three-dimensional orbits of the disk stars and checks the randomness of their orbital phases. The clockwise-disk stars are found to have modest orbital eccentricities. ------------------------------------------------------------------------ Email : iginsbur@cfa.harvard.edu Title : The Fate of Former Companions to Hypervelocity Stars Originating at the Galactic Center Author(s): Idan Ginsburg \& Abraham Loeb Paper : MNRAS, Oct 2005, accepted EPrint : astro-ph/0510574 Abstract: The hypervelocity star SDSS J090745.0+024507 in the halo of the Milky Way galaxy most likely originated from the breakup of a binary star system by the central black hole, SgrA*. We examine the fate of former binary companions to similar hypervelocity stars (HVSs) by simulating 600 different binary orbits around SgrA* with a direct N-body integration code. For some orbital parameters, the binary breakup process leads to HVSs with ejection velocities that are almost an order of magnitude larger than the velocity observed for SDSS J090745.0+024507. The former companion stars populate highly eccentric orbits which resemble the observed orbits for some of the stars nearest to SgrA*. ------------------------------------------------------------------------ Email : javier.rodriguez-goicoechea@lra.ens.fr Title : Warm Water Vapor around Sagittarius B2 Author(s): Jose Cernicharo, Javier R. Goicoechea, Juan R. Pardo, Andres Asensio-Ramos Institute: (1) Departamento de Astrofisica Molecular e Infrarroja, IEM, CSIC, Serrano 121, 28006, Madrid, Spain (2) Instituto de Astrofisica de Canarias, E-38205, La Laguna, Tenerife, Spain Paper : ApJ, Jan 2006, in press EPrint : astro-ph/0601336 Abstract: In the region of Sgr B2 there are several condensations heated externally by nearby hot stars. Therefore H_2O far-IR lines are expected to probe only an external low-density and high temperature section of these condensations, whereas millimeter-wave lines can penetrate deeper into them where the density is higher and T_k lower. We have conducted a study combining H_2O lines in both spectral regions. First, Infrared Space Observatory observations of several H_2O thermal lines seen in absorption toward Sgr B2(M) at a spectral resolution of 35 km s^-1 have been analyzed. Second, an IRAM-30m telescope map of the para-H_2O 3_13-2_20 line at 183.31 GHz, seen in emission, has also been obtained and analyzed. The H_2O lines seen in absorption are optically thick and are formed in the outermost gas of the condensations in front of the far-IR continuum sources. They probe a maximum visual extinction of 5 to 10 mag. Radiative transfer models indicate that these lines are quite insensitive to temperature and gas density, and that IR photons from the dust play a dominant role in the excitation of the involved H_2O rotational levels. In order to get the physical conditions of the absorbing gas we have also analyzed the CO emission toward Sgr B2(M). We conclude, based on the observed CO J=7-6 line at 806.65 GHz with the \textitCaltech Submillimeter Observatory, and the lack of emission from the far-IR CO lines, that the gas density has to be lower than 10^4 cm^-3. Using the values obtained for the kinetic temperature and gas density from OH, CO, and other molecular species, we derive a water column density of (9+/-3)*10^16 cm^-2 in the absorbing gas. Hence, the water vapor abundance in this region, \chi(H_2O), is (1-2)*10^-5. The relatively low H_2O/OH abundance ratio in the region, 2-4, is a signature of UV photon dominated surface layers traced by far-IR observations. As a consequence the temperature of the absorbing gas is high, T_K 300-500 K, which allows very efficient neutral-neutral reactions producing H_2O and OH. On the other hand, the 183.31 GHz data provide a much better spatial and spectral resolution than the far-IR ISO data. This maser line allows to trace water deeper into the cloud, i.e., the inner, denser (n(H_2)>=10^5-6 cm^-3) and colder (T_k 40 K) gas. The emission is very strong toward the cores. The derived water vapor abundance for this component is a few*10^-7. There is also moderate extended emission around Sgr B2 main condensations, a fact that supports the water vapor abundance derived from far-IR H_2O lines for the outer gas. ------------------------------------------------------------------------ Email : profumo@caltech.edu Title : Dark Matter and the CACTUS Gamma-Ray Excess from Draco Author(s): Stefano Profumo and Marc Kamionkowski Institute: (1) California Institute of Technology, Pasadena, CA 91125, USA Paper : astro-ph/0601249 EPrint : astro-ph/0601249 Abstract: The CACTUS atmospheric Cherenkov telescope collaboration recently reported a gamma-ray excess from the Draco dwarf spheroidal galaxy. Draco features a very low gas content and a large mass-to-light ratio, suggesting as a possible explanation annihilation of weakly interacting massive particles (WIMPs) in the Draco dark-matter halo. We show that with improved angular resolution, future measurements can determine whether the halo is cored or cuspy, as well as its scale radius. We find the relevant WIMP masses and annihilation cross sections and show that supersymmetric models can account for the required gamma-ray flux. We compute for these supersymmetric models the resulting Draco gamma-ray flux in the GLAST energy range and the rates for direct neutralino detection and for the flux of neutrinos from neutralino annihilation in the Sun. We also discuss the possibility that the bulk of the signal detected by CACTUS comes from direct WIMP annihilation to two photons and point out that a decaying-dark-matter scenario for Draco is not compatible with the gamma-ray flux from the Galactic center and in the diffuse gamma-ray background. ------------------------------------------------------------------------ Email : tal.alexander@weizmann.ac.il Title : Resonant relaxation near a massive black hole: the stellar distribution and gravitational wave sources Author(s): Clovis Hopman and Tal Alexander Institute: Faculty of Physics, Weizmann Institute of Science, POB 26, Rehovot 76100, Israel Paper : ApJ, submitted EPrint : astro-ph/0601161 Abstract: Resonant relaxation (RR) of orbital angular momenta occurs near massive black holes (MBHs) where the stellar orbits are nearly Keplerian and so do not precess significantly. The resulting coherent torques efficiently change the magnitude of the angular momenta and rotate the orbital inclination in all directions. As a result, many of the tightly bound stars very near the MBH are rapidly destroyed by falling into the MBH on low-angular momentum orbits, while the orbits of the remaining stars are efficiently randomized. We solve numerically the Fokker-Planck equation in energy for the steady state distribution of a single mass population with a RR sink term. We find that the steady state current of stars, which sustains the accelerated drainage close to the MBH, can be <= than 10 times larger than that due to non-coherent 2-body relaxation alone. RR mostly affects tightly bound stars, and so it increases only moderately the total tidal disruption rate, which is dominated by stars originating from less bound orbits farther away. We show that the event rate of gravitational wave (GW) emission from inspiraling stars, originating much closer to the MBH, is dominated by RR dynamics. The GW event rate depends on the uncertain efficiency of RR. The efficiency indicated by the few available simulations implies rates <= 10 times higher than those predicted by 2-body relaxation, which would improve the prospects of detecting such events by future GW detectors, such as LISA. However, a higher, but still plausible RR efficiency can lead to the drainage of all tightly bound stars and strong suppression of GW events from inspiraling stars. We apply our results to the Galactic MBH, and show that the observed dynamical properties of stars there are consistent with RR. ------------------------------------------------------------------------ (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 ========================================================================