======================================================================== 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. 11 Jul 25, 2006 Recently submitted papers: -------------------------- 1) Ejection of Hyper-Velocity Stars from the Galactic Centre by Intermediate-Mass Black Holes (Baumgardt et al., MNRAS) 2) Guaranteed and Prospective Galactic TeV Neutrino Sources (Kistler & Beacom, astro-ph/0607082) 3) Young stars in the Galactic Centre: a potential intermediate-mass star origin (Dray et al., MNRAS) 4) X-ray Radiation from the Annihilation of Dark Matter at the Galactic Center (Bergstroem et al., astro-ph/0607327) 5) The K-band Spectrum of The Hot Star in IRS 8: An Outsider in the Galactic Center? (Geballe et al., ApJ) ------------------------------------------------------------------------ Email : holger@astro.uni-bonn.de Title : Ejection of Hyper-Velocity Stars from the Galactic Centre by Intermediate-Mass Black Holes Author(s): H. Baumgardt^1, A. Gualandris^2, 3 and S. Portegies Zwart^2, 3 Institute: ^1Argelander Institute for Astronomy, University of Bonn, Auf dem Huegel 71, 53121 Bonn, Germany, ^2Astronomical Institute ``Anton Pannekoek,'', University of Amsterdam, Kruislaan 403, Netherlands, ^3Section Computational Science, University of Amsterdam, Kruislaan 403, Netherlands, Paper : MNRAS in press EPrint : astro-ph/0607455 Abstract: We have performed N-body simulations of the formation of hyper-velocity stars (HVS) in the centre of the Milky Way due to inspiralling intermediate-mass black holes (IMBHs). We considered IMBHs of different masses, all starting from circular orbits at an initial distance of 0.1 pc. We find that the IMBHs sink to the centre of the Galaxy due to dynamical friction, where they deplete the central cusp of stars. Some of these stars become HVS and are ejected with velocities sufficiently high to escape the Galaxy. Since the HVS carry with them information about their origin, in particular in the moment of ejection, the velocity distribution and the direction in which they escape the Galaxy, detecting a population of HVS will provide insight in the ejection processes and could therefore provide indirect evidence for the existence of IMBHs. Our simulations show that HVS are generated in short bursts which last only a few Myrs until the IMBH is swallowed by the supermassive black hole (SMBH). HVS are ejected almost isotropically, which makes IMBH induced ejections hard to distinguish from ejections due to encounters of stellar binaries with a SMBH. After the HVS have reached the galactic halo, their escape velocities correlate with the distance from the Galactic centre in the sense that the fastest HVS can be found furthest away from the centre. The velocity distribution of HVS generated by inspiralling IMBHs is also nearly independent of the mass of the IMBH and can be quite distinct from one generated by binary encounters. Finally, our simulations show that the presence of an IMBH in the Galactic centre changes the stellar density distribution inside r<0.02 pc into a core profile, which takes at least 100 Myrs to replenish. ------------------------------------------------------------------------ Email : kistler@mps.ohio-state.edu Title : Guaranteed and Prospective Galactic TeV Neutrino Sources Author(s): Matthew D. Kistler John F. Beacom Paper : astro-ph/0607082 EPrint : astro-ph/0607082 Abstract: Recent observations, particularly from the HESS Collaboration, have revealed rich Galactic populations of TeV gamma-ray sources, including a collection unseen in other wavelengths. Many of these gamma-ray spectra are well measured up to 10 TeV, where low statistics make observations by air Cerenkov telescopes difficult. To understand these mysterious sources, especially at much higher energies-where a cutoff should eventually appear-new techniques are needed. We point out the following: (1) For a number of sources, it is very likely that pions, and hence TeV neutrinos, are produced; (2) As a general point, neutrinos should be a better probe of the highest energies than gamma rays, due to increasing detector efficiency; and (3) For several specific sources, the detection prospects for km^3 neutrino telescopes are very good, 1-10 events/year, with low atmospheric neutrino background rates above reasonable energy thresholds. Such signal rates, as small as they may seem, will allow neutrino telescopes to powerfully discriminate between models for the Galactic TeV sources, with important consequences for our understanding of cosmic-ray production. ------------------------------------------------------------------------ Email : lmd16@astro.le.ac.uk Title : Young stars in the Galactic Centre: a potential intermediate-mass star origin Author(s): L. M. Dray^1, A. R. King^1 and M. B. Davies^2 Institute: 1. Theoretical Astrophysics Group, University of Leicester, Leicester, LE1 7RH, UK, 2. Lund Observatory, Box 43, SE-221 00 Lund, Sweden Paper : MNRAS accepted EPrint : astro-ph/0607470 Abstract: There has been recent speculation (Davies & King 2005) that the cores of intermediate-mass stars stripped of their envelopes by tidal interaction with the supermassive black hole in the Galactic centre could form a population observationally similar to the so-called Sgr A^* cluster or `S' stars, which have close eccentric orbits around the hole. We model the evolution of such stars, and show that the more luminous end of the population may indeed appear similar to young B stars within the observational limits of the Galactic Centre region. Whether some or all of these cluster stars can be accounted for in this manner depends strongly on the assumed IMF of the loss cone stars and the scattering rate. If most of the observed stars are in fact scattered from the Galactic Centre inner cusp region itself then the population of 20 to current observational limits may be reproduced. However, this only works if the local relaxation time is small and relies on the cusp stars themselves being young, i.e. it is dependent on some star formation being possible in the central few parsecs. Conversely, we obtain a possible constraint on the tidal stripping rate of `normal'-IMF stars if there are not to be red stars visible in the Sgr A^* cluster. ------------------------------------------------------------------------ Email : malc@physto.se Title : X-ray Radiation from the Annihilation of Dark Matter at the Galactic Center Author(s): Lars Bergstroem Malcolm Fairbairn Lidia Pieri Paper : astro-ph/0607327 EPrint : astro-ph/0607327 Abstract: The existing and upcoming multiwavelength data from the Galactic Center suggest a comparative study in order to propose or rule out possible models which would explain the observations. In this paper we consider the X-ray synchrotron and the gamma-ray emission due to Kaluza Klein Dark Matter and define a set of parameters for the shape of the Dark Matter halo which is consistent with the observations. We show that for this class of models the existing Chandra X-ray data is more restrictive than the constraints on very high energy gamma-rays coming from HESS. ------------------------------------------------------------------------ Email : tgeballe@gemini.edu Title : The K-band Spectrum of The Hot Star in IRS 8: An Outsider in the Galactic Center? Author(s): T. R. Geballe(1), F. Najarro(2), F. Rigaut(1), J.-R. Roy(1) Institute: (1) Gemini Observatory, 670 N. A'ohoku Place, Hilo, Hawaii 96720 USA; tgeballe@gemini.edu (2) Instituto de Estructura de la Materia, CSIC, Serrano 121, 29006 Madrid, Spain Paper : ApJ, 10 Nov 2006, in press EPrint : astro-ph/0607550 Abstract: Using adaptive optics at the Gemini North telescope we have obtained a K-band spectrum of the star near the center of the luminous Galactic center bowshock IRS 8, as well as a spectrum of the bowshock itself. The stellar spectrum contains emission and absorption lines characteristic of an O5-O6 giant or supergiant. The wind from such a star is fully capable of producing the observed bowshock. However, both the early spectral type and the apparently young age of the star, if it is single, mark it as unique among hot stars within one parsec of the center. ------------------------------------------------------------------------ (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 ========================================================================