======================================================================== 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. 21 Dec 30, 2010 Dear reader, This is the last GCNEWSflash containing abstracts. We have noticed that we cannot compete with other abstract services causing the number of submssions drop significantly during the last few years. We will however keep the flash available for announcements and meeting info to inform our subscribers. Happy 2011 Recently submitted papers: -------------------------- 1) Mapping photodissociation and shocks in the vicinity of Sgr A^* (Amo-Baladron et al., A&A) 2) The Jet in the Galactic Center: An Ideal Laboratory for Magnetohydrodynamics and General Relativity (Falcke et al., ) 3) Binaries migrating in a gaseous disk: Where are the Galactic center binaries? (Baruteau & Lin, ApJ) ------------------------------------------------------------------------ Email : arancha@damir.iem.csic.es Title : Mapping photodissociation and shocks in the vicinity of Sgr A^* Author(s): M. A. Amo-Baladron(1) J. Martin-Pintado(1) S. Martin(2,3) Institute: (1) Centro de Astrobiologia (CSIC/INTA), Ctra. de Torrejon a Ajalvir km 4, E-28850, Torrejon de Ardoz, Madrid, Spain, (2) European Southern Observatory, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago 19, Chile, (3) Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA, Paper : A&A, Oct 2010, accepted Web : http://arxiv.org/abs/1011.3271 Abstract: We study the chemistry in the harsh environments of galactic nuclei using the nearest one, the Galactic center (GC). We have obtained maps of the molecular emission within the central five arcminutes (12 pc) of the GC in selected molecular tracers: SiO(2-1), HNCO(5_0,5-4_0,4), and the J = 1 -> 0 transition of H^13CO^+, HN^13C, and C^18O at an angular resolution of 30'' (1.2 pc). The mapped region includes the circumnuclear disk (CND) and the two surrounding giant molecular clouds (GMCs) of the Sgr A complex, known as the 20 and 50 km s^-1 molecular clouds. Additionally, we simultaneously observed the J = 2 -> 1 and J = 3 -> 2 transitions of SiO toward selected positions to estimate the physical conditions of the molecular gas using the large velocity gradient approximation. The SiO(2-1) emission shows all the molecular features identified in previous studies, covering the same velocity range as the H^13CO^+(1-0) emission, which also presents a similar distribution. In contrast, HNCO(5-4) emission appears in a narrow velocity range mostly concentrated in the 20 and 50 km s^-1 GMCs. A similar trend follows the HN^13C(1-0) emission. The HNCO column densities and fractional abundances present the highest contrast, with difference factors of >=60 and 28, respectively. Their highest values are found toward the cores of the GMCs, whereas the lowest ones are measured at the CND. SiO abundances do not follow this trend, with high values found toward the CND, as well as the GMCs. By comparing our abundances with those of prototypical Galactic sources we conclude that HNCO, similar to SiO, is ejected from grain mantles into gas-phase by nondissociative C-shocks. This results in the high abundances measured toward the CND and the GMCs. However, the strong UV radiation from the Central cluster utterly photodissociates HNCO as we get closer to the center, whereas SiO seems to be more resistant against UV-photons or it is produced more efficiently by the strong shocks in the CND. This UV field could be also responsible for the trend found in the HN^13C abundance. We discuss the possible connections between the molecular gas at the CND and the GMCs using the HNCO/SiO, SiO/CS, and HNCO/CS intensity ratios as probes of distance to the Central cluster. In particular, the HNCO/SiO intensity ratio is proved to be an excellent tool for evaluating the distance to the center of the different gas components. ------------------------------------------------------------------------ Email : h.falcke@astro.ru.nl Title : The Jet in the Galactic Center: An Ideal Laboratory for Magnetohydrodynamics and General Relativity Author(s): Heino Falcke, Sera Markoff, Geoffrey C. Bower, Charles F. Gammie, Monika Mo'scibrodzka, Dipankar Maitra Abstract: Of all possible black hole sources, the event horizon of the Galactic Center black hole, Sgr A*, subtends the largest angular scale on the sky. It is therefore a prime candidate to study and image plasma processes in strong gravity and it even allows imaging of the shadow cast by the event horizon. Recent mm-wave VLBI and radio timing observations as well as numerical GRMHD simulations now have provided several breakthroughs that put Sgr A* back into the focus. Firstly, VLBI observations have now measured the intrinsic size of Sgr A* at multiple frequencies, where the highest frequency measurements have approached the scale of the black hole shadow. Moreover, measurements of the radio variability show a clear time lag between 22 GHz and 43 GHz. The combination of size and timing measurements, allows one to actually measure the flow speed and direction of magnetized plasma at some tens of Schwarzschild radii. This data strongly support a moderately relativistic outflow, consistent with an accelerating jet model. This is compared to recent GRMHD simulation that show the presence of a moderately relativistic outflow coupled to an accretion flow Sgr A*. Further VLBI and timing observations coupled to simulations have the potential to map out the velocity profile from 5-40 Schwarzschild radii and to provide a first glimpse at the appearance of a jet-disk system near the event horizon. Future submm-VLBI experiments would even be able to directly image those processes in strong gravity and directly confirm the presence of an event horizon. ------------------------------------------------------------------------ Email : jrcuadra@puc.cl Title : Binaries migrating in a gaseous disk: Where are the Galactic center binaries? Author(s): C. Baruteau, J. Cuadra and D.N.C. Lin Paper : ApJ, Nov 2010, in press Web : http://arxiv.org/abs/1011.0360 Abstract: The massive stars in the Galactic center inner arcsecond share analogous properties with the so-called Hot Jupiters. Most of these young stars have highly eccentric orbits, and were probably not formed in-situ. It has been proposed that these stars acquired their current orbits from the tidal disruption of compact massive binaries scattered toward the proximity of the central supermassive black hole. Assuming a binary star formed in a thin gaseous disk beyond 0.1 pc from the central object, we investigate the relevance of disk-satellite interactions to harden the binding energy of the binary, and to drive its inward migration. A massive, equal-mass binary star is found to become more tightly wound as it migrates inwards toward the central black hole. The migration timescale is very similar to that of a single-star satellite of the same mass. The binary's hardening is caused by the formation of spiral tails lagging the stars inside the binary's Hill radius. We show that the hardening timescale is mostly determined by the mass of gas inside the binary's Hill radius, and that it is much shorter than the migration timescale. We discuss some implications of the binary's hardening process. When the more massive (primary) components of close binaries eject most their mass through supernova explosion, their secondary stars may attain a range of eccentricities and inclinations. Such processes may provide an alternative unified scenario for the origin of the kinematic properties of the central cluster and S-stars in the Galactic center as well as the high velocity stars in the Galactic halo. ------------------------------------------------------------------------ (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 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ========================================================================