======================================================================== 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. 11 Jul 6, 2009 Recently submitted papers: -------------------------- 1) On the dissolution of star clusters in the Galactic centre. I. Circular orbits (Ernst et al., MNRAS) 2) Two Rapidly Variable Galactic X-ray Transients Observed with Chandra, XMM and Suzaku (Heinke et al., ApJ) 3) Star Formation at the Galactic Center (Fatuzzo & Melia, PASP) 4) LUNASKA Experiment Observational Limits on UHE Neutrinos from Centaurus A and the Galactic Center (James et al.) ------------------------------------------------------------------------ Email : aernst@ari.uni-heidelberg.de Title : On the dissolution of star clusters in the Galactic centre. I. Circular orbits Author(s): A. Ernst^1,2, A. Just^1, R. Spurzem^1 Institute: (1) Astronomisches Rechen-Institut/Zentrum fuer Astronomie der Universitaet Heidelberg, Moenchhofstrasse 12-14, 69120 Heidelberg, Germany, (2) Max-Planck-Institut fuer Astronomie, Koenigstuhl 17, 69117 Heidelberg, Germany Paper : MNRAS, 2009, accepted EPrint : 0906.4459 Abstract: We present N-body simulations of dissolving star clusters close to galactic centres. For this purpose, we developed a new N-body program called nbody6gc based on Aarseth's series of N-body codes. We describe the algorithm in detail. We report about the density wave phenomenon in the tidal arms which has been recently explained by Kuepper et al. (2008). Standing waves develop in the tidal arms. The wave knots or clumps develop at the position, where the emerging tidal arm hits the potential wall of the effective potential and is reflected. The escaping stars move through the wave knots further into the tidal arms. We show the consistency of the positions of the wave knots with the theory in Just et al. (2009). We also demonstrate a simple method to study the properties of tidal arms. By solving many eigenvalue problems along the tidal arms, we construct numerically a 1D coordinate system whose directio n is always along a principal axis of the local tensor of inertia. Along this coordinate system, physical quantities can be evaluated. The half-mass or dissolution times of our models are almost independent of the particle number which indicates that two-body relaxation is not the dominant mechanism leading to the dissolution. This may be a typical situation for many young star clusters. We propose a classification scheme which sheds light on the dissolution mechanism. ------------------------------------------------------------------------ Email : cheinke@phys.ualberta.ca Title : Two Rapidly Variable Galactic X-ray Transients Observed with Chandra, XMM and Suzaku Author(s): C. O. Heinke(1), J. A. Tomsick(2), F. Yusef-Zadeh(3), J. E. Grindlay(4) Institute: (1) University of Alberta, Dept. of Physics, 11322-89 Avenue, Edmonton AB, T6G 2G7, Canada (2) Space Sciences Laboratory, 7 Gauss Way, University of California, Berkeley, CA 94720-7450, USA (3) Northwestern University, Dept. of Physics & Astronomy, 2145 Sheridan Rd., Evanston, IL 60208, USA (4) Harvard University, Dept. of Astronomy, 60 Garden Street, Cambridge, MA 02138, USA Paper : ApJ, August 2009, in press Abstract: We have identified two moderately bright, rapidly variable transients in new and archival X-ray data near the Galactic center. Both objects show strong, flaring variability on timescales of tens to thousands of seconds, evidence of N_H variability, and hard spectra. XMMU J174445.5-295044 is seen at 2-10 keV fluxes of 3*10^-11 to <10^-12 ergs cm^-2 s^-1, with N_H at or above 5*10^22 cm^-2, by XMM, Chandra, and Suzaku. A likely 2MASS counterpart with K_S=10.2 shows colors indicative of a late-type star. CXOU J174042.0-280724 is a likely counterpart to the fast hard transient IGR J17407-2808. Chandra observations find F_X(2-10 keV) 10^-12 ergs cm^-2 s^-1, with large N_H variations (from 2*10^22 to >2*10^23 cm^-2). No 2MASS counterpart is visible, to K_S>13. XMMU J174445.5-295044 seems likely to be a new symbiotic star or symbiotic X-ray binary, while CXOU J174042.0-280724 is more mysterious, likely an unusual low-mass X-ray binary. ------------------------------------------------------------------------ Email : fatuzzo@xavier.edu Title : Star Formation at the Galactic Center Author(s): Marco Fatuzzo,^1 and Fulvio Melia^2 Institute: (1) Physics Department, Xavier University, Cincinnati, OH 45207 (2) Department of Physics and Steward Observatory, The University of Arizona, Tucson, Arizona 85721 Paper : PASP, V121, 880, 585 EPrint : 0906.2917 Abstract: Molecular clouds at the Galactic center (GC) have environments considerably different from their disk counterparts. The GC may therefore provide important clues about how the environment affects star formation. Interestingly, while the inner 50 parsecs of our Galaxy include a remarkable population of high-mass stars, the initial mass function (IMF) appears to be consistent with a Salpeter slope down to 1 M_o. We show here that the loss of turbulent pressure due to ambipolar diffusion and the damping of Alfven and fast MHD waves can lead to the formation of dense condensations exceeding their Jeans limit. The fragmentation and subsequent collapse of these condensations is similar to the diffusion-driven protostellar collapse mechanism expected to occur within nearby ``regular" molecular clouds. As such, a Salpeter IMF at the GC is not surprising, though the short dynamical timescales associated with the GC molecular clouds may help explain the lower star formation efficiency observed from this region. ------------------------------------------------------------------------ Email : rprother@physics.adelaide.edu.au Title : LUNASKA Experiment Observational Limits on UHE Neutrinos from Centaurus A and the Galactic Center Author(s): C. W. James^1,*, R. J. Protheroe^1, R. D. Ekers^2, J. Alvarez-Muniz,^3 R. A. McFadden^4,2, C. J. Phillips^2, P. Roberts^2, J. D. Bray^1,2 Abstract: We present the first observational limits to the ultra-high energy (UHE) neutrino flux from the Galactic Center, and from Centaurus A which is the nearest active galactic nucleus. These results are based on our ``Lunar UHE Neutrino Astrophysics using the Square Kilometer Array'' (LUNASKA) project experiments at the Australia Telescope Compact Array (ATCA). We also derive limits for the previous experiments and compare these limits with expectations for acceleration and super-heavy dark matter models of the origin of UHE cosmic rays. ------------------------------------------------------------------------ (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, Masaaki Sakano, Feng Yuan - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - For Abstract submission please follow the instructions which are at http://www.aoc.nrao.edu/~gcnews/home/submission.shtml ========================================================================