======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 29, No. 1 Dec 12, 2008 Recently submitted papers: -------------------------- 1) HD 271791: an extreme supernova run-away B star escaping from the Galaxy (Przybilla et al., ApJ) 2) The chemical abundances in the Galactic Centre from the atmospheres of Red Supergiants (Davies et al., ApJ) 3) A Catalog of 24 micron Sources Toward the Galactic Center (Hinz et al., ApJS) 4) Red giant stellar collisions in the Galactic Centre (Dale et al., MNRAS) 5) Obtaining the diffusion coefficient for cosmic ray propagation in the Galactic Centre Ridge through time-dependent simulations of their gamma -ray emission (Dimitrakoudis et al., Astropart.) ------------------------------------------------------------------------ Email : Norbert.Przybilla@sternwarte.uni-erlangen.de Title : HD 271791: an extreme supernova run-away B star escaping from the Galaxy Author(s): Norbert Przybilla(1), M. Fernanda Nieva(1,2), Ulrich Heber(1) and Keith Butler(3) Institute: (1) Dr. Karl Remeis-Observatory, University of Erlangen-Nuremberg, Sternwartstr. 7, D-96049 Bamberg, Germany (2) Max-Planck-Institute for Astrophysics, Karl-Schwarzschild-Str. 1, D-85741 Garching, Germany (3) University Observatory, Scheinerstr. 1, D-86179 Munich, Germany Paper : ApJ, 684, L103 Abstract: Hyper-velocity stars (HVSs) were first predicted by theory to be the result of the tidal disruption of a binary system by a super-massive black hole (SMBH) that accelerates one component to beyond the Galactic escape velocity (the Hills mechanism). Because the Galactic centre hosts such a SMBH it is the suggested place of origin for HVSs. However, the SMBH paradigm has been challenged recently by the young HVS HD 271791 because its kinematics point to a birthplace in the metal-poor rim of the Galactic disc. Here we report the atmosphere of HD 271791 to indeed show a sub-solar iron abundance along with an enhancement of the alpha -elements, indicating capture of nucleosynthesis products from a supernova or a more energetic hypernova. This implies that HD 271791 is the surviving secondary of a massive binary system disrupted in a supernova explosion. No such run-away star has ever been found to exceed the Galactic escape velocity, hence HD 271791 is the first hyper-runaway star. Such a run-away scenario is an alternative to the Hills mechanism for the acceleration of some HVSs with moderate velocities. The observed chemical composition of HD 271791 puts invaluable observational constraints on nucleosynthesis in a supernova from the core-collapse of a very massive star (M_ZAMS > 55 M_o), which may be observed as a gamma-ray burst of the long-duration/soft-spectrum type. ------------------------------------------------------------------------ Email : eurobenjy@googlemail.com Title : The chemical abundances in the Galactic Centre from the atmospheres of Red Supergiants Author(s): Ben Davies(1,2), Livia Origlia(3), Rolf-Peter Kudritzki(4), Don F. Figer(2), R. Michael Rich(5), Francisco Najarro(6) Institute: (1) School of Physics \& Astronomy, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, U.K. (2) Chester F. Carlson Center for Imaging Science, Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester NY, 14623, USA (3) INAF-Osservatorio Astronomico di Bologna, Via Ranzani 1, I-40127 Bologna, Italy (4) Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI, 96822, USA (5) Department of Physics and Astronomy, University of California at Los Angeles, Los Angeles, CA 90095-1547, USA (6) Instituto de Estructura de la Materia, Consejo Superior de Investigaciones Cientificas, Calle Serrano 121, 28006 Madrid, Spain. Paper : ApJ Nov 2008, in press EPrint : 0811.3179 Web : http://xxx.lanl.gov/abs/0811.3179 Abstract: The Galactic Centre (GC) has experienced a high degree of recent star-forming activity, as evidenced by the large number of massive stars currently residing there. The relative abundances of chemical elements in the GC may provide insights into the origins of this activity. Here, we present high-resolution H-band spectra of two Red Supergiants in the GC (IRS 7 and VR 5-7), and in combination with spectral synthesis we derive abundances for Fe and C, as well as other alpha -elements Ca, Si, Mg Ti and O. We find that the C-depletion in VR 5-7 is consistent with the predictions of evolutionary models of RSGs, while the heavy depletion of C and O in IRS 7's atmosphere is indicative of deep mixing, possibly due to fast initial rotation and/or enhanced mass-loss. Our results indicate that the current surface Fe/H content of each star is slightly above Solar. However, comparisons to evolutionary models indicate that the initial Fe/H ratio was likely closer to Solar, and has been driven higher by H-depletion at the stars' surface. Overall, we find alpha /Fe ratios for both stars which are consistent with the thin Galactic disk. These results are consistent with other chemical studies of the GC, given the precision to which abundances can currently be determined. We argue that the GC abundances are consistent with a scenario in which the recent star-forming activity in the GC was fuelled by either material travelling down the Bar from the inner disk, or from the winds of stars in the inner Bulge - with no need to invoke top-heavy stellar Initial Mass Functions to explain anomalous abundance ratios. ------------------------------------------------------------------------ Email : jhinz@as.arizona.edu Title : A Catalog of 24 micron Sources Toward the Galactic Center Author(s): J. L. Hinz(1), G. H. Rieke(1), F. Yusef-Zadeh(2), J. Hewitt(2), Z. Balog(1,3), M. Block(1) Institute: (1) Steward Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721 (2) Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Rd., Evanston, IL 60208 (3) On leave from the Department of Optics and Quantum Electronics, University of Szeged, H-6720, Szeged, Hungary Paper : ApJS, accepted EPrint : 0811.1557 Abstract: We present a 1.5\arcdeg*8\arcdeg (220*1195 pc) Multiband Imaging Photometer for Spitzer 24 micron image of the Galactic Center and an accompanying point source list. This image is the highest spatial resolution (6'' 0.25 pc) and sensitivity map ever taken across the GC at this wavelength, showing the emission by warm dust in unprecedented detail. Over 120,000 point sources are identified in this catalog with signal-to-noise ratios greater than five and flux densities from 0.6 mJy to 9 Jy. ------------------------------------------------------------------------ Email : jim@ig.cas.cz Title : Red giant stellar collisions in the Galactic Centre Author(s): James. E. Dale^1, Melvyn B. Davies^1, Ross P. Church^1,2, Marc Freitag^3 Institute: ^1 Lund Observatory, Box 43, SE-221 00, Lund, Sweden ^2 School of Mathematical Sciences, Monash University, Victoria 3800, Australia, ^3Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA, UK Paper : MNRAS, 2008, in press EPrint : 0811.3111 Abstract: We show that collisions with stellar-mass black holes can partially explain the absence of bright giant stars in the Galactic Centre, first noted by Genzel et al, 1996. We show that the missing objects are low-mass giants and AGB stars in the range 1-3 M_o. Using detailed stellar evolution calculations, we find that to prevent these objects from evolving to become visible in the depleted K bands, we require that they suffer collisions on the red giant branch, and we calculate the fractional envelope mass losses required. Using a combination of Smoothed Particle Hydrodynamic calculations, restricted three-body analysis and Monte Carlo simulations, we compute the expected collision rates between giants and black holes, and between giants and main-sequence stars in the Galactic Centre. We show that collisions can plausibly explain the missing giants in the 10.5