======================================================================== 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. 6 Mar 25, 2009 Recently submitted papers: -------------------------- 1) Dynamical evolution of the young stars in the Galactic center: N-body simulations of the S-stars (Perets et al., ApJ) 2) Evidence for X-ray synchrotron emission from simultaneous mid-IR to X-ray observations of a strong Sgr A* flare (Dodds-Eden et al., ApJ) 3) Interstellar extinction and long-period variables in the Galactic centre (Schultheis et al., A&A) 4) Gas infall towards Sgr A* from the clumpy circumnuclear disk (Montero-Castano et al., ApJ) 5) The warped young stellar disc in the Galactic centre (Subr et al., A&A) 6) Tidal effects on small bodies by massive black holes (Kostic et al., A&A) ------------------------------------------------------------------------ Email : hagai.perets@weizmann.ac.il Title : Dynamical evolution of the young stars in the Galactic center: N-body simulations of the S-stars Author(s): Hagai B. Perets (1), Alessia Gualandris (2), Gabor Kupi (1), David Merritt (2), Tal Alexander (1) Institute: (1) Weizmann Institute of Science, POB 26, Rehovot 76100, Israel (2) Department of Physics and Center for Computational Relativity and Gravitation, Rochester Institute of Technology, Rochester, NY 14623 Paper : ApJ, submitted Abstract: We use N-body simulations to study the evolution of the orbital eccentricities of stars deposited near (<=sssim0.05 pc) the Milky Way massive black hole (MBH), starting from initial conditions motivated by two competing models for their origin: formation in a disk followed by inward migration; and exchange interactions involving a binary star. The first model predicts modest eccentricities, lower than those observed in the S-star cluster, while the second model predicts higher eccentricities than observed. The N-body simulations include a dense cluster of 10M_o stellar black holes (SBHs), expected to accumulate near the MBH by mass segregation. Perturbations from the SBHs tend to randomize the stellar orbits, partially erasing the dynamical signatures of their origin. The eccentricities of the initially highly eccentric stars evolve, in 20 Myr (the S-star lifespan), to a distribution that is consistent at the 95% level with the observed eccentricity distribution. In contrast, the eccentricities of the initially more circular orbits fail to evolve to the observed values in 20 Myr, arguing against the disk migration scenario. We find that 20%-30% of the S-stars are tidally disrupted by the MBH over their lifetimes, and that the S-stars are not likely to be ejected as hypervelocity stars outside the central 0.05 pc by close encounters with stellar black holes. ------------------------------------------------------------------------ Email : katie@mpe.mpg.de Title : Evidence for X-ray synchrotron emission from simultaneous mid-IR to X-ray observations of a strong Sgr A* flare Author(s): K. Dodds-Eden (1), D. Porquet (2), G. Trap (3,4), E. Quataert (5), X. Haubois (6), S. Gillessen (1), N. Grosso (2), E. Pantin (3,7), H. Falcke (8,9), D. Rouan (6), R. Genzel (1,10), G. Hasinger (1), A. Goldwurm (3,4), F. Yusef-Zadeh (11), Y. Clenet (6), S. Trippe (1), P.-O. Lagage (3,7), H. Bartko (1), F. Eisenhauer (1), T. Ott (1), T. Paumard (6), G. Perrin (6), F. Yuan (12), T.K. Fritz (1), L. Mascetti (1) Institute: (1) Max Planck Institut fuer Extraterrestrische Physik, Postfach 1312, D-85741, Garching, Germany. (2) Observatoire astronomique de Strasbourg, Universite de Strasbourg, CNRS, INSU, 11 rue de l'Universite, F-67000 Strasbourg, France (3) CEA, IRFU, Service d'Astrophysique, Centre de Saclay, F-91191 Gif-surYvette, France. (4) AstroParticule et Cosmologie (APC), 10 rue Alice Domont et Leonie Duquet, F-75205 Paris, France. (5) Department of Astronomy, University of California, Berkeley, 601 Campbell Hall, Berkeley, CA 94720-3411. (6) LESIA, Observatoire de Paris, CNRS, UPMC, Universite Paris Diderot; 5 Place Jules Janssen, 92190 Meudon, France (7) Laboratoire AIM, CEA - Centre de Saclay, F-91191 Gif-sur-Yvette, France. (8) Department of Astrophysics, Institute for Mathematics, Astrophysics and Particle Physics, Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands (9) ASTRON, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands (10) Department of Physics, University of California, Berkeley, 366 Le Comte Hall, Berkeley, CA 94720-7300 (11) Department of Physics and Astronomy, Northwestern University, Evanston, Il. 60208 (12) Shanghai Astronomical Observatory, Shanghai 200030, China Paper : ApJ, accepted EPrint : 09033416 Abstract: This paper reports measurements of Sgr A* made with NACO in L'-band (3.80 micron ), Ks-band (2.12 micron ) and H-band (1.66 micron ) and with VISIR in N-band (11.88 micron ) at the ESO VLT The Very Large Telescope (VLT) at the European Southern Observatory (ESO) on Paranal, Chile: Program IDs 179.B-0261(A) and 60.A-9234(A)., as well as with XMM-Newton at X-ray (2-10 keV) wavelengths. On 4 April, 2007, a very bright flare was observed from Sgr A* simultaneously at L'-band and X-ray wavelengths. No emission was detected using VISIR. The resulting SED has a blue slope ( beta > 0 for nu L_ nu \propto nu ^ beta , consistent with nu L_ nu \propto nu ^0.4) between 12 micron and 3.8 micron. For the first time our high quality data allow a detailed comparison of infrared and X-ray light curves with a resolution of a few minutes. The IR and X-ray flares are simultaneous to within 3 minutes. However the IR flare lasts significantly longer than the X-ray flare (both before and after the X-ray peak) and prominent substructures in the 3.8 micron light curve are clearly not seen in the X-ray data. From the shortest timescale variations in the L'-band lightcurve we find that the flaring region must be no more than 1.2 R_S in size. The high X-ray to infrared flux ratio, blue nu L_ nu slope MIR to L'-band, and the soft nu L_ nu spectral index of the X-ray flare together place strong constraints on possible flare emission mechanisms. We find that it is quantitatively difficult to explain this bright X-ray flare with inverse Compton processes. A synchrotron emission scenario from an electron distribution with a cooling break is a more viable scenario. ------------------------------------------------------------------------ Email : mathias@obs-besancon.fr Title : Interstellar extinction and long-period variables in the Galactic centre Author(s): M. Schultheis(1,2), K. Sellgren(3), S. Ramirez(4), S. Stolovy(5), S. Ganesh(6), I.S. Glass (7) and L. Girardi(8) Institute: (1) Observatoire de Besan\ccon, 41bis, avenue de l'Observatoire, F-25000 Besan\ccon, France (2) Institut d'Astrophysique de Paris, CNRS, 98bis Bd Arago, F-75014 Paris, France (3) Astronomy Department, Ohio State University, Columbus, OH 43210, USA (4) IPAC, Caltech, Pasadena, CA 91125, USA (5) Spitzer Science Center, Caltech, Pasadena, CA 91125, USA (6) Physical Research Laboratory, Astronomy \& Astrophysics Division, Ahmedabad, India (7) South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa (8) Osservatorio di Padua, Italy Paper : A&A 495, 157 Abstract: We derive a new map of the interstellar extinction near the Galactic Centre (GC), extending to much higher values of A_V than previously available, and use the results obtained to better characterise the long-period variable star population of the region. We take the Spitzer IRAC catalogue of GC point sources (Ramirez et al. 2008) and combine it with new isochrones (Marigo et al. 2008) to derive extinctions based on the photometry of red giants and asymptotic giant branch (AGB) stars. We apply it to deredden the LPVs found by Glass et al. (2001) near the GC (Glass-LPVs). We make period-magnitude diagrams and compare them to those from other regions of different metallicity. Our new extinction map of the GC region covers 2.0 DEG * 1.4 DEG (280 * 200 pc at a distance of 8 kpc). The Glass-LPVs follow well-defined period-luminosity relations (PL) in the IRAC filter bands at 3.6, 4.5, 5.8, and 8.0 micron . The period-luminosity relations are similar to those in the Large Magellanic Cloud, suggesting that the PL relation in the IRAC bands is universal. We use ISOGAL data to derive mass-loss rates and find for the Glass-LPV sample some correlation between mass loss and pulsation period, as expected theoretically. Theoretical isochrones for a grid of different metallicities and ages are able to reproduce this relation. The GC has an excess of high-luminosity and long-period LPVs compared to the bulge, which supports previous suggestions that it contains a younger stellar population. ------------------------------------------------------------------------ Email : mmontero@cfa.harvard.edu Title : Gas infall towards Sgr A* from the clumpy circumnuclear disk Author(s): Maria Montero-Castano (1,2,3), Robeson M. Herrnstein (4) and Paul T.P. Ho (1,3) Institute: (1) Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (2) Departamento de Astrofisica, Facultad de Ciencias Fisicas, Universidad Complutense de Madrid, 28040-Madrid, Spain. (3) Institute of Astronomy and Astrophysics, Academia Sinica, P.O. Box 23141, Taipei 106, Taiwan. (4) Department of Astronomy, Columbia University, 550 West 120th Street, New York, NY 10027 Paper : Apj, accepted EPrint : 0903.0886 Abstract: We present the first large-scale mosaic performed with the Submillimeter Array (SMA^5) in the Galactic center. We have produced a 25-pointing mosaic, covering a 2' * 2' area around Sgr A*. We have detected emission from two high-density molecular tracers, HCN(4-3) and CS(7-6), the latter never before reported in this region. The data have an angular resolution of 4.6'' * 3.1'', and the spectral window coverage is from -180 km s^-1 to 1490 km s^-1 for HCN(4-3) and from -1605 km s^-1 to 129 km s^-1 for CS(7-6). Both molecular tracers present a very clumpy distribution along the circumnuclear disk (CND), and are detected with a high signal-to-noise ratio in the southern part of the CND, while they are weaker towards the northern part. Assuming that the clumps are as close to the Galactic center as their projected distances, they are still dense enough to be gravitationally stable against the tidal shear produced by the supermassive black hole. Therefore, the CND is a non-transient structure. This geometrical distribution of both tracers suggests that the southern part of the CND is denser than the northern part. Also, by comparing the HCN(4-3) results with HCN(1-0) results we can see that the northern and the southern parts of the CND have different excitation levels, with the southern part warmer than the northern. Finally, we compare our results with those obtained with the detection of NH_3, which traces the warmer and less dense material detected in the inner cavity of the CND. We suggest that we are detecting the origin point where a portion of the CND becomes destabilized and approaches the dynamical center of the Milky Way, possibly being impacted by the southern streamer and heated on its way inwards. ------------------------------------------------------------------------ Email : subr@sirrah.troja.mff.cuni.cz Title : The warped young stellar disc in the Galactic centre Author(s): L. Subr (1,2,3) J. Schovancova (1), P. Kroupa (3) Institute: (1) Faculty of Mathematics and Physics, Charles University, V Holesovickach 2, CZ-18000 Praha, Czech Republic (2) Astronomical Institute, Academy of Sciences, Bocni II, CZ-14131 Praha, Czech Republic (3) Argelander Institute for Astronomy (AIfA), Auf dem Huegel 71, D-53121 Bonn, Germany Paper : A&A, 2009, in press EPrint : 0812.1567 Abstract: Within the central parsec of the Galaxy, several dozen young stars orbiting a central supermassive black hole are observed. A subset of these stars forms a coherently rotating disc. Other observations reveal a massive molecular torus that lies at a radius 1.5 pc from the centre. In this paper we consider the gravitational influence of the molecular torus upon the stars of the stellar disc. We derive an analytical formula for the rate of precession of individual stellar orbits and we show that it is highly sensitive to the orbital semi-major axis and inclination with respect to the plane of the torus, as well as to the mass of the torus. Assuming that both the stellar disc and the molecular torus are stable on the timescale > 6Myr, we constrain the mass of the torus and its inclination with respect to the young stellar disc. We further suggest that all young stars observed in the Galactic centre may have a common origin in a single coherently rotating structure with an opening angle <= 5 deg, which was partially destroyed (warped) during its lifetime by the gravitational influence of the molecular torus. ------------------------------------------------------------------------ Email : uros.kostic@fmf.uni-lj.si Title : Tidal effects on small bodies by massive black holes Author(s): Uros Kostic (1) and Andrej Cadez (1) and Massimo Calvani (2) and Andreja Gomboc (1) Institute: (1) Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia (2) INAF - Astronomical Observatory of Padova, Vicolo Osservatorio 5, 35122 Padova, Italy Paper : A&A, 496, 307-315 (2009) Web : http://astro.ago.uni-lj.si/bh/uploads/Main/aa_tidal2.pdf Abstract: The compact radio source Sagittarius A* (Sgr A*) at the centre of our Galaxy harbours a supermassive black hole, whose mass ( 3.7 * 10^6 M_o) has been measured from stellar orbital motions. Sgr A* is therefore the nearest laboratory where super-massive black hole astrophysics can be tested, and the environment of black holes can be investigated. Since it is not an active galactic nucleus, it also offers the possibility of observing the capture of small objects that may orbit the central black hole. We study the effects of the strong gravitational field of the black hole on small objects, such as a comet or an asteroid. We also explore the idea that the flares detected in Sgr A* might be produced by the final accretion of single, dense objects with mass of the order of 10^20 g, and that their timing is not a characteristic of the sources, but rather of the space-time of the central galactic black hole in which they are moving. The problem of tidal disruption of small objects by a black hole is studied numerically, using ray-tracing techniques, in a Schwarzschild background. We find that tidal effects are strong enough to melt sufficiently massive, solid objects, and present calculations of the temporal evolution of the light curve of infalling objects as a function of various parameters. Our modelling of tidal disruption suggests that during tidal squeezing, the conditions for synchrotron radiation can be met. We show that the light curve of a flare can be deduced from dynamical properties of geodesic orbits around black holes and that it depends only weakly on the physical properties of the source. ------------------------------------------------------------------------ (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 ========================================================================