======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 18, No. 3 Jul 1, 2004 Correction to GCFLASH Vol. 18, No. 2: ------------------------------------- In the paper submitted by Maillard et al. (2nd item) the EPrint reference was missing. Title : The nature of the Galactic Center source IRS 13 revealed by high spatial resolution in the infrared EPrint : astro-ph/0404450 Recently submitted papers: -------------------------- 1) A cool disk in the Galactic Center? (Liu & Meyer-Hofmeister, A&A) 2) In situ acceleration in the Galactic Center Arc (Lieb & Birk, 2004A&A...419..161L) 3) Dust Embedded Sources at the Galactic Center 2 to 4 micron imaging and spectroscopy in the central parsec (Moultaka et al., A&A) 4) Kinematic and structural analysis of the Minispiral in the Galactic Center from BEAR spectro-imagery (Paumard et al., A&A) 5) Life and Death of Young Dense Star Clusters near the Galactic Center (Zwart et al., ) 6) A Jet Model of the Galactic Center Nonthermal Radio Filaments (Yusef-Zadeh et al., Cancun) ------------------------------------------------------------------------ Email : bifang@MPA-Garching.MPG.DE Title : A cool disk in the Galactic Center? Author(s): B.F. Liu , F. Meyer and E. Meyer-Hofmeister Institute: Max-Planck-Institut fuer Astrophysik, Karl- Schwarzschildstr. 1, D-85740 Garching, Germany Paper : A&A, v421p659-666 EPrint : astro-ph/0403460 Abstract: We study the possibility of a cool disk existing in the Galactic Center in the framework of the disk-corona evaporation/condensation model. Assuming an inactive disk near the gravitational capture distance left over from an earlier evolutionary stage, a hot corona should form above the disk since there is a continuous supply of hot gas from stellar winds of the close-by massive stars. We study the interaction between the disk and the corona. Whether the cool disk can survive depends on the mass exchange between disk and corona which is determined by the energy and pressure balance. If evaporation is the dominant process and the rate is larger than the Bondi accretion rate in the Galactic Center, the disk will be depleted within a certain time and no persistent disk will exist. On the other hand, if the interaction results in hot gas steadily condensing into the disk, an inactive cool disk with little gas accreting towards the central black hole might survive in the Galactic Center. For this case we further investigate the Bremsstrahlung radiation from the hot corona and compare it with the observed X-ray luminosity. Our model shows that, for standard viscosity in the corona ( alpha =0.3), the mass evaporation rate is much higher than the Bondi accretion rate and the coronal density is much larger than that inferred from Chandra observations. An inactive disk can not survive such strong evaporation. For small viscosity ( alpha < 0.07) we find condensation solutions. But detailed coronal structure computations show that in this case there is too much X-ray radiation from the corona to be in agreement with the observations. From this modeling we conclude that there should be no thin/inactive disk presently in the Galactic Center. However we do not exclude that the alternative non-radiative model of Nayakshin (2004) might instead be realized in nature and shortly discuss this question. ------------------------------------------------------------------------ Email : lieb@usm.uni-muenchen.de Title : In situ acceleration in the Galactic Center Arc Author(s): S. Lieb, H. Lesch and G.T. Birk Paper : 2004A&A...419..161L EPrint : astro-ph/0402595 Abstract: For the nonthermal radio emission of the Galactic Center Arc in situ electron acceleration is imperative. The observed radio spectrum can be modeled by a transport equation for the relativistic electrons which includes particle acceleration by electric fields, momentum diffusion via scattering by magnetohydrodynamical turbulence and energy losses by synchrotron radiation. The accelerating electric fields can be regarded as a natural consequence of multiple reconnection events, caused by the interaction between a molecular cloud and the Arc region. The radio spectrum and even the recently detected 150 GHz emission, explicitely originating from the interaction regions of a molecular cloud with the magnetized Arc, can be explained in terms of quasi-monoenergetically distributed relativistic electrons with a typical energy of about 10 GeV accelerated in stochastically distributed magnetic reconnection zones. ------------------------------------------------------------------------ Email : moultaka@ph1.uni-koeln.de Title : Dust Embedded Sources at the Galactic Center 2 to 4 micron imaging and spectroscopy in the central parsec Author(s): J. Moultaka^1, A. Eckart^1, T. Viehmann^1, N. Mouawad^1, C. Straubmeier^1, T. Ott^2, R. Schoedel^1 Institute: 1) I Physikalishes Institut, Zuelpicher Str. 77, 50937 Koeln, Germany, 2) Max Planck Institut fuer extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany, email: moultaka@ph1.uni-koeln.de Paper : A&A, in press EPrint : astro-ph/0406154 Abstract: We present the first L-band spectroscopic observations for a dozen stellar sources in the central 0.5 pc of the GC stellar cluster that are bright in the 2-4 micron wavelength domain. The L-band data were taken with ISAAC at the VLT UT1 (Antu). With the aid of additional K-band spectroscopic data, we derive optical depth spectra of the sources after fitting their continuum emission with a single reddened blackbody continuum. We also derive intrinsic source spectra by correcting the line of sight extinction via the optical depth spectrum of a late type star that is most likely not affected by local dust emission or extinction at the Galactic Center. The good agreement between the two approaches shows that the overall variation of the line-of-sight extinction across the central 0.5 pc is \Delta AK<=0.5 mag. The extinction corrected spectra of the hot He-stars are in good agreement with pure Rayleigh Jeans continuum spectra. The intrinsic spectra of all other sources are in agreement with continuum emission and absorption features due to the dust in which they are embedded. We interprete both facts as evidence that a significant amount of the absorption takes place within the central parsec of the Galactic Center and is most likely associated with the individual sources there. We find absorption features at 3.0 mu m, 3.4 mu m, and 3.48 mu m wavelength. Correlations between all three features show that they are very likely to arise in the ISM of the central 0.5 pc. Spacially highly variable hydrogen emission lines seen towards the individual sources give evidence of the complex density and temperature structure of the mini-spiral. In the cases of the sources with featureless K-band spectra like IRS 21 and IRS 1W, they are consistent with massive hot stars embedded in the bow shock created by their motion through the dust and gas of the mini-spiral. It is likely that the bow shock scenario may be applicable to most of the dust embedded sources in the central stellar cluster. Spectroscopy of high MIR-excess sources 0.5'' north of the IRS 13 complex is largely consistent with them being YSOs. However, a bow-shock nature of these sources cannot be excluded. The L-band spectrum at the location of SgrA*, is consistent with that of a hot O-type star, such as S2, which was very close to Sgr A* at the time of our observations. ------------------------------------------------------------------------ Email : paumard@iap.fr Title : Kinematic and structural analysis of the Minispiral in the Galactic Center from BEAR spectro-imagery Author(s): Thibaut Paumard^1, and Jean-Pierre Maillard^1, and Mark Morris^2 Institute: (1) Institut d'astrophysique de Paris (CNRS), 98b Blvd Arago, 75014 Paris, France (2) University of California, Los Angeles, Div. of Astronomy, Dept of Physics and Astronomy, Los Angeles, CA 90095-1562, USA Paper : A&A, 2004, in press EPrint : astro-ph/0405197 Abstract: Integral field spectroscopy of the inner region of the Galactic Center, over a field of roughly 40''*40'' was obtained at 2.06 micron (HeI) and 2.16 micron (Br gamma ) using BEAR, an imaging Fourier Transform Spectrometer, at spectral resolutions respectively of 52.9 km s^-1 and 21.3 km s^-1, and a spatial resolution of 0.5''. The analysis of the data was focused on the kinematics of the gas flows, traditionally called the ``Minispiral'', concentrated in the neighborhood of the central black hole, Sgr A*. From the decomposition into several velocity components (up to four) of the line profile extracted at each point of the field, velocity features were identified. Nine distinguishable structures are described: the standard Northern Arm, Eastern Arm, Bar, Western Arc, and five additional, coherently-moving patches of gas. From this analysis, the Northern Arm appears not limited, as usually thought, to the bright, narrow North-South lane seen on intensity images, but it consists instead of a weak, continuous, triangular-shaped surface, drawn out into a narrow stream in the vicinity of Sgr A* where it shows a strong velocity gradient, and a bright western rim. The Eastern Arm is split into three components (a Ribbon and a Tip, separated by a cavity, and an elongated feature parallel to the Ribbon: the Eastern Bridge). We also report extinction of some interstellar structures by other components, providing information on their relative position along the line of sight. A system of Keplerian orbits can be fitted to most of the Northern Arm, and the bright rim of this feature can be interpreted in terms of line-of-sight orbit crowding caused by the warping of the flowing surface at the western edge facing Sgr A*. These results lead to a new picture of the gas structures in Sgr A West, in which large-scale gas flows and isolated gas patches coexist in the gravitational field of the central Black Hole. The question of the origin of the ionized gas is addressed and a discussion of the lifetime of these features is presented. ------------------------------------------------------------------------ Email : spz@science.uva.nl Title : Life and Death of Young Dense Star Clusters near the Galactic Center Author(s): Simon F. Portegies Zwart^1 Stephen L. W. McMillan^2 Holger Baumgardt^3 Institute: (1) Astronomical Institute `Anton Pannekoek', University of Amsterdam, Kruislaan 403, the Netherlands and Institute for Computer Science, University of Amsterdam, Kruislaan 403 (2) Department of Physics, Drexel University, Philadelphia, PA 19104, USA (3) Institute of Advanced Physical and Chemical Research RIKEN, 2-1 Hirosawa, Wako-shi, Saitama 351-019, Japan EPrint : astro-ph/0403147 Abstract: We discuss the structural change and degree of mass segregation of young dense star clusters within about 100 pc of the Galactic center. In our calculations, which are performed with GRAPE-6, the equations of motion of all stars and binaries are calculated accurately but the external potential of the Galaxy is solved (semi)analytically. The simulations are preformed to model the Arches star cluster. We find that star clusters with are less strongly perturbed by the tidal field and dynamical friction are much stronger affected by mass segregation; resulting in a significant pile-up of massive stars in the cluster center. At an age of about 3.5 Myr more than 90 per cent of the stars more massive than 10 \msun are concentrated within the half-mass radius of the surviving cluster. Star clusters which are strongly perturbed by the tidal field of the parent Galaxy are much less affected by mass segregation. ------------------------------------------------------------------------ Email : zadeh@northwestern.edu Title : A Jet Model of the Galactic Center Nonthermal Radio Filaments Author(s): F. Yusef-Zadeh^1, A. Koenigl^2 Institute: (1) Dept Physics and Astronomy, Northwestern University, Evanston, IL 60208 (2) Department of Astronomy and Astrophysics, University of Chicago, 5640 S. Ellis Ave., Chicago, IL 60637 Paper : Cancun Workshop (in press) EPrint : astro-ph/0403201 Abstract: Protostellar sources in star forming regions are responsible for driving jets with flow velocities ranging between 300 and 400 km s^-1. This class of jets consists of highly collimated outflows which include thermal knots with number densities estimated to be greater than that of their ambient medium. On the other hand, extragalactic FR I jets consist of light fluid with low Mach number burrowing through a denser medium as the magnetized jets radiate nonthermal emission. Both protostellar and extragalactic jets are believed to be launched by accretion disks. Here we consider a jet model in which the characteristics common to both protostellar and extragalactic jets are used to explain the origin of nonthermal filaments in the Galactic center region. We argue that these filaments are analogous to FR I extragalactic sources but are launched by embedded young stars or clusters of stars in star-forming regions. ------------------------------------------------------------------------ (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 ========================================================================