======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 22, No. 2 Aug 24, 2005 Recently submitted papers: -------------------------- 1) Three-body encounters in the Galactic centre: the origin of the hypervelocity star SDSS J090745.0+024507 (Gualandris et al., MNRAS) 2) A Hot Helium Plasma in the Galactic Center Region (Belmont et al., ApJ) 3) Stellar Winds and Embedded Star Formation in the Galactic Center Quintuplet and Arches Clusters: Multifrequency Radio Observations (Lang et al., AJ) 4) X-raying the super star clusters in the Galactic center (Oskinova, JENAM) 5) VLT L-band mapping of the Galactic Center IRS 3-IRS 13 region Evidence for new Wolf-Rayet type stars (Moultaka et al., astro-ph/0507161) 6) Magnetic-Reconnection and Current-Sheet Model for the Radio Arc and Threads in the Galactic Center (Sofue et al., ) 7) Stellar Processes Near the Massive Black Hole in the Galactic Center (Alexander, iPhysics Reports) 8) Hot and Diffuse Clouds near the Galactic Center Probed by Metastable H3+ (Oka et al., ApJ) ------------------------------------------------------------------------ Email : alessiag@science.uva.nl Title : Three-body encounters in the Galactic centre: the origin of the hypervelocity star SDSS J090745.0+024507 Author(s): Alessia Gualandris ^1, Simon Portegies Zwart^1 and Michael S. Sipior^1, ^1 Astronomical Institute 'Anton Pannekoek', and Section Computational Science, University of Amsterdam, the Netherlands, Paper : MNRAS, in press EPrint : astro-ph/0507365 Abstract: Hills (1988) predicted that runaway stars could be accelerated to velocities larger than 1000 km/sec by dynamical encounters with the supermassive black hole (SMBH) in the Galactic center. The recently discovered hypervelocity star SDSS J090745.0+024507 (hereafter HVS) is escaping the Galaxy at high speed and could be the first object in this class. With the measured radial velocity and the estimated distance to the HVS, we trace back its trajectory in the Galactic potential. Assuming it was ejected from the center, we find that a 2\masyr proper motion is necessary for the star to have come within a few parsecs of the SMBH. We perform three-body scattering experiments to constrain the progenitor encounter which accelerated the HVS. As proposed by Yu \& Tremaine (2003), we consider the tidal disruption of binary systems by the SMBH and the encounter between a star and a binary black hole, as well as an alternative scenario involving intermediate mass black holes. We find that the tidal disruption of a stellar binary ejects stars with a larger velocity compared to the encounter between a single star and a binary black hole, but has a somewhat smaller ejection rate due to the greater availability of single stars. ------------------------------------------------------------------------ Email : belmont@cea.fr Title : A Hot Helium Plasma in the Galactic Center Region Author(s): R. Belmont(1), M. Tagger(1), M. Muno(2), M. Morris(2), and S. Cowley(2,3) Institute: (1) CEA Service d'Astrophysique, UMR "AstroParticules et Cosmologie", Orme des Merisiers, 91191 Gif-sur-Yvette, France (2) Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA (3) UMR "Astro-Particules et Cosmologie" Paper : ApJ, 2005, accepted for publication EPrint : astro-ph/0508480 Abstract: Recent X-ray observations by the space mission Chandra confirmed the astonishing evidence for a diffuse, hot, thermal plasma at a temperature of 9. 10^7 K ( 8 keV) found by previous surveys to extend over a few hundred parsecs in the Galactic Centre region. This plasma coexists with the usual components of the interstellar medium such as cold molecular clouds and a soft ( 0.8 keV) component produced by supernova remnants, and its origin remains uncertain. First, simple calculations using a mean sound speed for a hydrogen-dominated plasma have suggested that it should not be gravitationally bound, and thus requires a huge energy source to heat it in less than the escape time. Second, an astrophysical mechanism must be found to generate such a high temperature. No known source has been identified to fulfill both requirements. Here we address the energetics problem and show that the hot component could actually be a gravitationally confined helium plasma. We illustrate the new prospects this opens by discussing the origin of this gas, and by suggesting possible heating mechanisms. ------------------------------------------------------------------------ Email : cornelia-lang@uiowa.edu Title : Stellar Winds and Embedded Star Formation in the Galactic Center Quintuplet and Arches Clusters: Multifrequency Radio Observations Author(s): Cornelia C. Lang, Kelsey E. Johnson, W. M. Goss, Luis F. Rodriguez Institute: (1) Department of Physics \& Astronomy, University of Iowa, Iowa City, IA 52245 (2) Department of Astronomy, University of Virginia, Charlottesville, VA 22903 (3) National Radio Astronomy Observatory, Socorro, NM 87801 (4) Centro de Radioastronomi a y Astrofi sica, UNAM, Campus Morelia, Apdo. Postal 3-72, Morelia, Michoacan 58089, Mexico Paper : AJ, November 2005, in press EPrint : astro-ph/0508178 Abstract: A multi-frequency, multi-configuration study has been made of the compact radio sources in the Galactic Center Quintuplet and Arches stellar clusters using the Very Large Array. Ten radio sources have been detected in the Quintuplet cluster. The majority of these radio sources have rising spectral indices and are positionally coincident with young massive stars that are known to have powerful stellar winds. We conclude that the three most compact of these sources are produced by stellar wind emission; thus, mass-loss rates can be derived and have an average value of 3*10^-5 \msun \yr. The remainder of the sources are likely to be a combination of stellar wind emission and free-free emission from surrounding ionized gas. In three cases, the radio sources have no stellar counterpart and the radio emission is thought to arise from compact or ultra-compact \ion H2 regions. If so, these sources would be the first detections of embedded massive stars to be discovered in the Galactic center clusters. The radio nebula associated with the Pistol star resembles the nebula surrounding the LBV star \eta Carina and may be related to the stellar wind of the Pistol star. Ten compact radio sources are detected in the Arches cluster and are interpreted to be stellar wind sources, consistent with previous findings. Several of the sources show moderate variability (10-30%) in their flux density, possibly related to a nonthermal component in the wind emission. A number of radio sources in both clusters have X-ray counterparts, which have been interpreted to be the shocked, colliding winds of massive binary systems. ------------------------------------------------------------------------ Email : lida@astro.physik.uni-potsdam.de Title : X-raying the super star clusters in the Galactic center Author(s): L. M. Oskinova Institute: (1) Astrophysik, Universitaet Potsdam, Germany Paper : JENAM 2005 conf.proc. EPrint : astro-ph/0507693 Web : http://www.astro.physik.uni-potsdam.de/~ftp/oskinova/x-ray_ss c.ps Abstract: The Galactic center harbors some of the most massive star clusters known in the Galaxy: the Arches and the Quintuplet. Based on the Chandra observations of these clusters (PI: Wang) which recently became public, I discuss the X-ray emission from the massive stars in these clusters. Confirming the general trend for Wolf-Rayet (WR) stars being X-ray dim, none of them is detected in the Quintuplet cluster. The most massive star known in the Galaxy, the Pistol star, is also not detected, invoking questions regarding the proposed binary nature of this object. X-ray emission in the Arches cluster is dominated by three stellar point sources. All three sources as well as the cluster's diffuse radiation show strong emission at 6.4-6.7 keV, indicating the presence of fluorescenting cool material. The Arches point sources may be identified as colliding wind binaries, albeit other possibilities cannot be ruled out. ------------------------------------------------------------------------ Email : moultaka@ph1.uni-koeln.de Title : VLT L-band mapping of the Galactic Center IRS 3-IRS 13 region Evidence for new Wolf-Rayet type stars Author(s): J. Moultaka^1, A. Eckart^1, R. Schoedel^1, T. Viehmann^1, \& F. Najarro^2 Institute: (1) 1) I Physikalishes Institut, Zuelpicher Str. 77, 50937 Koeln, Germany, 2) Instituto de Estructura de la Materia, Consejo Superior de Investigaciones Cientificas, CSIC, Serrano 121, E-28006, Madrid, Spain, \emailemail: moultaka@ph1.uni-koeln.de Paper : astro-ph/0507161 EPrint : astro-ph/0507161 Abstract: This paper presents L-band ISAAC and NAOS/CONICA (VLT) spectroscopic observations of the IRS 3-IRS 13 Galactic Center region. The ISAAC data allowed us to build the first spectroscopic data cube of the region in the L-band domain. Using the L-band spectrum of the extinction along the line of sight towards the GC derived in a previous paper (Moultaka et al. 2004), it was also possible to correct the cube for the foreground extinction. Maps of the water ice and hydrocarbon absorption line strength were then derived. These maps are important diagnostics of the interstellar and circumstellar medium because water ices are observed in molecular clouds while hydrocarbons are usually good tracers of the diffuse ISM. These maps support our previous results that the absorption features most probably occur in the local Galactic center medium and can be associated with the individual sources. Moreover, turbulence seems to affect the studied region of the minispiral, which appears like a mixture of a dense and diffuse medium. Comparison of the concentrations of ice and hydrocarbon absorptions around IRS 13E, IRS 6E, and IRS 2, with similar concentrations at the location of the extended continuum emission around IRS 3, suggests that these sources might present outflows interacting with the surrounding ISM. It was also possible to derive Br alpha and Pf gamma emission line maps. The results suggest that the physical conditions of the ISM are not uniform in the observed region of the minispiral especially at the edges of the minicavity. The emission line maps allowed us to find three sources with broad lines corresponding to an FWHM deconvolved line width of about 1100 km/s and moving towards us with a radial velocity of about -300km/s. These sources are most probably new Wolf-Rayet type stars located in projection to the north and west of IRS 3. Their derived radial velocities and proper motions show that only two of them might belong to the two rotating disks of young stars reported by Genzel et al. (2003) and Levin \& Beloborodov (2003). Previously, NAOS/CONICA (NACO) data allowed us to resolve the IRS13E3 region into two components E3N, and E3c (Eckart et al. 2004). The new spectroscopic NACO data show that E3c is a good candidate for a Wolf-Rayet type star. In addition, three sources (\eta, \zeta, and gamma ) out of the eight very red sources located in the IRS13N complex also presented in Eckart et al. (2004) have been resolved spectroscopically with NACO. The spectra presented in this paper show that the red colors of the sources are probably due to extended dust emission. ------------------------------------------------------------------------ Email : sofue@ioa.s.u-tokyo.ac.jp Title : Magnetic-Reconnection and Current-Sheet Model for the Radio Arc and Threads in the Galactic Center Author(s): Yoshiaki Sofue, Institute of Astronomy, University of Tokyo, Mitaka, Tokyo 181-0015, Hiromitsu Kigure, and Kazunari Shibata, Kwasan and Hida Observatory, Yamashina, Kyoto 607-8471 EPrint : astro-ph/0507568 Abstract: We propose a new mechanism to explain the radio Arc and threads in the Galactic Center by current sheets produced by local magnetic shears due to interaction of a moving cloud and the vertical field based on three-dimensional magneto-hydrodynamical simulations. Magnetic reconnetion and acceleration of cosmic-ray electrons in the current sheet will result in high contrast of radio emissivity inside and outside the Arc and threads.,, Key words: Galactic Center; Interstellar matter; Magnetic field; MHD simulation, For figures, see astro-ph/0507568 ------------------------------------------------------------------------ Email : tal.alexander@weizmann.ac.il Title : Stellar Processes Near the Massive Black Hole in the Galactic Center Author(s): Tal Alexander Institute: (1) Faculty of Physics, The Weizmann Institute of Science, PO Box 26, Rehovot 76100, Israel Paper : invited review, Physics Reports,2005, in press EPrint : astro-ph/0508106 Abstract: A massive black hole resides in the center of most, perhaps all galaxies. The one in the center of our home galaxy, the Milky Way, provides a uniquely accessible laboratory for studying in detail the connections and interactions between a massive black hole and the stellar system in which it grows; for investigating the effects of extreme density, velocity and tidal fields on stars; and for using stars to probe the central dark mass and to probe post-Newtonian gravity in the weak- and strong-field limits. Recent results, open questions and future prospects are reviewed in the wider context of the theoretical framework and physical processes that underlie them. ------------------------------------------------------------------------ Email : tgeballe@gemini.edu Title : Hot and Diffuse Clouds near the Galactic Center Probed by Metastable H3+ Author(s): Takeshi Oka, Thomas R. Geballe, Miwa Goto, Tomonori Usuda, Benjamin J. McCall, Institute: (1) Department of Astronomy and Astrophysics, and Department of Chemistry, The Enrico Fermi Institute, University of Chicago, Chicago, IL 60637 USA (2) Gemini Observatory, Hilo, Hawaii 96720 USA (3) Max Planck Institute for Astronomy, Heidelberg, Germany (4) Subaru Telescope, National Astronomical Observatory of Japan, Hilo, Hawaii 96720 USA (5) Department of Chemistry and Department of Astronomy, University of Illinois Urbana-Champaign, Urbana, IL 61801-3792 USA Paper : ApJ, in press EPrint : astro-ph/0507463 Abstract: We have observed a vast amount of high temperature (T 250 K) and low density (n 100 cm^-3) gas with a large velocity dispersion in the central 200 pc of the Galactic center (the Central Molecular Zone; CMZ) that has not previously been reported. We used the infrared spectrum of H_3^+ which is a sensitive probe of low density molecular gas. The observed large column density of H_3^+ in the (J, K) = (3, 3) metastable rotational level (361 K above the lowest (1, 1) level) gives evidence for high temperature, and the observed small column density in the (2, 2) level (210 K below (3, 3)), gives evidence for low density. This remarkable non-thermal rotational distribution is caused for a low density gas by the fact that the spontaneous emission from the (3, 3) level is rigorously forbidden by the selection rules while that for the (2, 2) -> (1, 1) transition has a short lifetime of 27 days, corresponding to a low critical density of 200 cm^-3. Observed H_3^+ spectrum toward the brightest infrared source GCS 3-2, one of the Quintuplet Stars, has been analyzed in detail. Of the observed total H_3^+ column density of 4.3 * 10^15 cm^-2, approximately 3.1 * 10^15 cm^-2 with high velocity dispersion is inferred to be in the CMZ while 1.2 * 10^15 cm^-2 is in the intervening spiral arms. Almost all of H_3^+ in the CMZ are in diffuse clouds with high temperature. About a half of the gas has a velocity of - 100 km s^-1 indicating that it is associated with the 180 pc Expanding Molecular Ring which approximately forms the boundary of the CMZ. The other half with lower velocities of - 50 km s^-1 and 0 km s^-1 is thought to be closer to the Galactic center. CO do not exist much in those clouds. The non-thermal rotational distribution of H_3^+ has also been observed toward 7 other infrared sources within 40 pc of the Galactic center indicating that the hot and diffuse gas is ubiquitous in the CMZ. The spectrum toward GC IRS 3 near Sgr A* shows presence of the hot and diffuse gas in the ``50 km s^-1 cloud", the complex of giant molecular clouds which plays a central role in the discussion of Sgr A* and its environment. The hot and diffuse gas has not been observed toward any of the dense and diffuse clouds in the Galactic disk where we have observed large column densities of H_3^+ had been observed. The large observed H_3^+ column density suggests an ionization rate in the CMZ which is an on the order of magnitude higher than in the diffuse interstellar medium in the Galactic disk if the C/H ratio is indeed as high as reported. Also the fact that the observed H_3^+ in the CMZ are almost all in diffuse clouds, along with the reported relatively low visual extinctions (A_V 25 - 40) and mass estimated from radio observations of molecules, indicate that dense clouds are more sparse than previously thought and the reported volume filling factor (f >= 0.1) is an overestimate by at least an order of magnitude. ------------------------------------------------------------------------ (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, Robin Herrnstein - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - For Abstract submission please follow the instructions which are at http://www.aoc.nrao.edu/~gcnews/home/submission.shtml ========================================================================