======================================================================== 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. 2 Jan 9, 2009 Recently submitted papers: -------------------------- 1) Simulations of the formation of stellar discs in the Galactic centre via cloud-cloud collisions (Hobbs & Nayakshin, MNRAS) 2) A New Secular Instability of Eccentric Stellar Disks Around Supermassive Black Holes,, with Application to the Galactic center (Madigan & Hopman, ApJL) 3) On compact super-massive objects without event horizon (Verozub, Il) 4) Explaining the Orbits of the Galactic Center S-Stars (Merritt et al., arXiv:0812.4517v1) 5) The Galactic Center 50-km s^-1 Molecular Cloud with An Expanding Shell (Tsuboi et al., PASJ) ------------------------------------------------------------------------ Email : alexander.hobbs@astro.le.ac.uk Title : Simulations of the formation of stellar discs in the Galactic centre via cloud-cloud collisions Author(s): Alexander Hobbs and Sergei Nayakshin Paper : MNRAS, Jan 2009, in press EPrint : 0809.3752 Web : http://www.astro.le.ac.uk/~aph11/papers/colliding_clouds.pdf Abstract: Young massive stars in the central parsec of our Galaxy are best explained by star formation within at least one, and possibly two, massive self-gravitating gaseous discs. With help of numerical simulations, we here consider whether the observed population of young stars could have originated from a large angle collision of two massive gaseous clouds at R 1 pc from \sgra. In all the simulations performed, the post-collision gas flow forms an inner, nearly circular gaseous disc and one or two eccentric outer filaments, consistent with the observations. Furthermore, the radial stellar mass distribution is always very steep, SIGMA _* \propto R^-2, again consistent with the observations. All of our simulations produce discs that are warped by between 30^o to 60^o, in accordance with the most recent observations. The 3D velocity structure of the stellar distribution is sensitive to initial conditions (e.g., the impact parameter of the clouds) and gas cooling details. For example, the runs in which the inner disc is fed intermittently with material possessing fluctuating angular momentum result in multiple stellar discs with different orbital orientations, contradicting the observed data. In all the cases the amount of gas accreted by our inner boundary condition is large, enough to allow \sgra to radiate near its Eddington limit over 10^5 years. This suggests that a refined model would have physically larger clouds (or a cloud and a disc such as the circumnuclear disc) colliding at a distance of a few parsecs rather than 1 parsec as in our simulations. ------------------------------------------------------------------------ Email : annmarie.madigan@gmail.com Title : A New Secular Instability of Eccentric Stellar Disks Around Supermassive Black Holes,, with Application to the Galactic center Author(s): Ann-Marie Madigan, Yuri Levin and Clovis Hopman Paper : ApJL, submitted EPrint : 0812.3395 Abstract: We identify a new secular instability of eccentric stellar disks around supermassive black holes. We show that retrograde precession of the stellar orbits, due to the presence of a stellar cusp, induces coherent torques that amplify deviations of individual orbital eccentricities from the average, and thus drive all eccentricities away from their initial value. We investigate the instability using N-body simulations, and show that it can drive individual orbital eccentricities to significantly higher or lower values on the order of a precession time-scale. This physics is relevant for the Galactic center, where massive stars are likely to form in eccentric disks around the SgrA* black hole. We show that the dynamical evolution of such a disk results in several of its stars acquiring high (1-e\ll 0.1) orbital eccentricity. Binary stars on such highly eccentric orbits would get tidally disrupted by the SgrA* black hole, possibly producing both S-stars near the black hole and high-velocity stars in the Galactic halo. ------------------------------------------------------------------------ Email : l.verozub@googlemail.com Title : On compact super-massive objects without event horizon Author(s): L. Verozub Institute: Kharkov National University - Kharkov, Ukraine Paper : Il Nouvo Cimento, accepted EPrint : arXiv:0806.3744 Abstract: This paper aims to show a possibility of the existence of super-massive compact objects with radii less than the Schwarzschild ones, which is one of the principal consequences of the author's geodesic-invariant gravitation equations. [Ann. Phys. (Berlin) , v. 17, p. 28 (2008)]. The physical interpretation of solutions of the equations is based on the conclusion that only an aggregate "space-time geometry + used reference frame" has a physical sense. ------------------------------------------------------------------------ Email : merritt@astro.rit.edu Title : Explaining the Orbits of the Galactic Center S-Stars Author(s): David Merritt(1), Alessia Gualandris(1), Seppo Mikkola(2) Institute: (1) Department of Physics and Center for Computational Relativity and Gravitation, Rochester Institute of Technology, Rochester, NY 14623 (2) Tuorla Observatory, University of Turku, Vaeisaelaentie 20, Piikkioe, Finland Paper : arXiv:0812.4517v1 Abstract: The young stars near the supermassive black hole at the galactic center follow orbits that are nearly random in orientation and that have an approximately ``thermal'' distribution of eccentricities, N( < e) e^2. We show that both of these properties are a natural consequence of a few million years' interaction with an intermediate-mass black hole (IBH), if the latter's orbit is mildly eccentric and if its mass exceeds approximately 1500 solar masses. Producing the most tightly-bound S-stars requires an IBH orbit with periastron distance less than about 10 mpc. Our results provide support for a model in which the young stars are carried to the galactic center while bound to an IBH, and are consistent with the hypothesis that an IBH may still be orbiting within the nuclear star cluster. ------------------------------------------------------------------------ Email : tsuboi@vsop.isas.jaxa.jp Title : The Galactic Center 50-km s^-1 Molecular Cloud with An Expanding Shell Author(s): Masato Tsuboi, Atsushi Miyazaki, Sachiko K. Okumura Institute: (1) Institute of Space and Astronautical Science, Sagamihara, Kanagawa 229-8510 (2) Department of Astronomy, the University of Tokyo, Bunkyo, Tokyo 113-0033 (3) Mizusawa VERA Observatory, National Astronomical Observatory, Mizusawa, Oshu, Iwate 023-0861 (4) National Astronomical Observatory, Mitaka, Tokyo, 181-8588 Paper : PASJ Feb 2009, in press Abstract: We performed high-resolution observation of the Galactic Center 50-km s ^-1 molecular cloud in the CS J = 1-0 line using the Nobeyama Millimeter Array (NMA). The molecular cloud mainly has three different spatial components with large velocity widths up to 60 km s ^-1. The northwest component is located at an apparent contact point to the Sgr A east shell and elongated along the boundary of the shell. The large velocity width of the component is responsible for the interaction with the Sgr A east shell. The molecular gas distribution in CS line emission is dissimilar to that observed previously in NH_3 line emissions. The appearances shows presumably the area of CS line emission enhanced by shock. The central and southwest components are located just out of the Sgr A east shell and far from it, respectively. However, these components have large velocity widths. We found a well-shaped circular molecular shell with expanding motion in the 50-km s^-1 molecular cloud. This is responsible for the large velocity width. The continuum source in the expanding molecular shell has a steep spectrum in the mm-wave although this is not identified in the previous 5-GHz map. This source may be an SNR with an ionized sheath. From the aspect ratio of the expanding molecular shell of 1.1, the magnetic field in/around the shell is estimated to be smaller than 100 mu Gauss. The weak magnetic field is consistent with on-going active star formation in the 50-km s ^-1 molecular cloud. The comparison among CS line emission, low frequency continuum, and millimeter continuum toward the 50-km s ^-1 molecular cloud suggests a face-on view of the Sgr A region. The molecular cloud is located in the Sgr A halo region. ------------------------------------------------------------------------ (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 ========================================================================