======================================================================== 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. 20 Oct 27, 2010 Recently submitted papers: -------------------------- 1) The Galactic Centre star S2 as a dynamical probe for intermediate-mass black holes (Gualandris et al., MNRAS) 2) Tidal breakup of binary stars at the Galactic Center. II., Hydrodynamic simulations (Antonini et al., ApJ) 3) The High-Density Ionized Gas in the Central Parsec of the Galaxy (Zhao et al., ApJ) ------------------------------------------------------------------------ Email : alessiag@MPA-Garching.MPG.DE Title : The Galactic Centre star S2 as a dynamical probe for intermediate-mass black holes Author(s): A. Gualandris, S. Gillessen, D. Merritt Paper : MNRAS, 2010, in press EPrint : 1004.2703 Abstract: We study the short-term effects of an intermediate mass black hole (IBH) on the orbit of star S2 (S02), the shortest-period star known to orbit the supermassive black hole (MBH) in the centre of the Milky Way. Near-infrared imaging and spectroscopic observations allow an accurate determination of the orbit of the star. Given S2's short orbital period and large eccentricity, general relativity (GR) needs to be taken into account, and its effects are potentially measurable with current technology. We show that perturbations due to an IBH in orbit around the MBH can produce a shift in the apoapsis of S2 that is as large or even larger than the GR shift. An IBH will also induce changes in the plane of S2's orbit at a level as large as one degree per period. We apply observational orbital fitting techniques to simulations of the S-cluster in the presence of an IBH and find that an IBH more massive than about 1000\msun at the distance of the S-stars will be detectable at the next periapse passage of S2, which will occur in 2018. ------------------------------------------------------------------------ Email : antonini@astro.rit.edu Title : Tidal breakup of binary stars at the Galactic Center. II., Hydrodynamic simulations Author(s): Fabio Antonini, James C. Lombardi Jr., David Merritt Institute: (1) Department of Physics and Center for Computational Relativity and Gravitation, Rochester Institute of Technology, 85 Lomb Memorial Drive, Rochester, NY 14623, USA (2) Department of Physics, Allegheny College, 520 North Main Street, Meadville, PA 16335, USA (3) Department of Physics and Center for Computational Relativity and Gravitation, Rochester Institute of Technology, 85 Lomb Memorial Drive, Rochester, NY 14623, USA Paper : ApJ, submitted Web : http://arxiv.org/abs/1008.5369 Abstract: In Paper I, we followed the evolution of binary stars as they orbited near the supermassive black hole (SMBH) at the Galactic center, noting the cases in which the two stars would come close enough together to collide. In this paper we replace the point-mass stars by fluid realizations, and use a smoothed-particle hydrodynamics (SPH) code to follow the close interactions. We model the binary components as main-sequence stars with initial masses of 1, 3 and 6 Solar masses, and with chemical composition profiles taken from stellar evolution codes. Outcomes of the close interactions include mergers, collisions that leave both stars intact, and ejection of one star at high velocity accompanied by capture of the other star into a tight orbit around the SMBH. For the first time, we follow the evolution of the collision products for many (> 100) orbits around the SMBH. Stars that are initially too small to be tidally disrupted by the SMBH can be puffed up by close encounters or collisions, with the result that tidal stripping occurs in subsequent periapse passages. In these cases, mass loss occurs episodically, sometimes for hundreds of orbits before the star is completely disrupted. Repeated tidal flares, of either increasing or decreasing intensity, are a predicted consequence. In collisions involving a low-mass and a high-mass star, the merger product acquires a high core hydrogen abundance from the smaller star, effectively resetting the nuclear evolution ``clock'' to a younger age. Elements like Li, Be and B that can exist only in the outermost envelope of a star are severely depleted due to envelope ejection during collisions and due to tidal forces from the SMBH. Tidal spin-up due to either a collision or tidal torque by the SMBH at periapsis can explain the observed high rotational velocity of HVS 8. However, in the absence of collisions, tidal spin-up of stars is only important in a narrow range of periapse distances, r_t/2<=sssim r_per <=sssim r_t with r_t the tidal disruption radius. We discuss the implications of these results for the formation of the S-stars and the hypervelocity stars ------------------------------------------------------------------------ Email : jzhao@cfa.harvard.edu Title : The High-Density Ionized Gas in the Central Parsec of the Galaxy Author(s): Jun-Hui Zhao, Ray Blundell, James M. Moran, Dennis Downes, Karl F. Schuster, Daniel P. Marrone Institute: (1) Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 78, Cambridge, MA 02138 (2) Institut de Radio Astronomie Millimetrique, 38406 Saint Martin d'Heres, France (3) Department of Astronomy \& Astrophysics, University of Chicago, Chicago, IL 60637 Paper : ApJ (2010), Vol. 722 (October 20, 2010), in press Abstract: We report a study of the H30 alpha line emission at 1.3 mm from the region around Sgr A* made with the Submillimeter Array at a resolution of 2'' over a field of 60'' (2 parsec) and a velocity range of -360 to +345 km/sec . This field encompasses most of the Galactic center's ``minispiral.'' With an isothermal homogeneous HII model, we determined the physical conditions of the ionized gas at specific locations in the Northern and Eastern Arms from the H30 alpha line data along with Very Large Array data from the H92 alpha line at 3.6 cm and from the radio continuum emission at 1.3 cm. The typical electron density and kinetic temperature in the minispiral arms are 3-21*10^4 cm^-3 and 5,000-13,000 K, respectively. The H30 alpha and H92 alpha line profiles are broadened due to the large velocity shear within and along the beam produced by dynamical motions in the strong gravitational field near Sgr A*. We constructed a 3D model of the minispiral using the orbital parameters derived under the assumptions that the gas flows are in Keplerian motion. The gas in the Eastern Arm appears to collide with the Northern Arm flow in the ``Bar" region, which is located 0.1-0.2 parsec south of and behind Sgr A*. Finally, a total Lyman continuum flux of 3*10^50 photons s^-1 is inferred from the assumption that the gas is photoionized and the ionizing photons for the high-density gas in the minispiral arms are from external sources, which is equivalent to 250 O9-type zero-age-main-sequence stars. ------------------------------------------------------------------------ (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 ========================================================================