======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 27, No. 10 Nov 21, 2007 CONFERENCE ANNONCEMENT: The Universe under the Microscope - Astrophysics at High Angular Resolution Web: https://www.ph1.uni-koeln.de/AHAR08 21-25 April 2008. Venue is the Physikzentrum Bad Honnef. High angular resolution techniques at infrared and centimeter to millimeter wavelengths have become of ever increasing importance for astrophysical research in the past decade. These new techniques will enable us  to address issues such as directly measuring the properties of exoplanets, imaging the surfaces of stars, examining stellar dynamics in extremely dense cluster cores, disentangling the processes at the bottom of black hole accretion flows in the jet launching region, or testing general relativity in the strong gravity regime near the event horizon of supermassive black holes. This conference aims at an interdisciplinary approach by bringing together astrophysicists from the three great branches of the field, instrumentation, observation, and theory, to discuss the current state of research and the possibilities offered by the next-generation instruments. Recently submitted papers: -------------------------- 1) The enigma of GCIRS 3. (Pott et al., A&A) 2) The most massive stars in the Arches cluster (Martins et al., A&A) 3) On the nature of the fast moving star S2 in the Galactic Center (Martins et al., ApJL) 4) Self-gravitating fragmentation of eccentric accretion disks (Alexander et al., ApJ) ------------------------------------------------------------------------ Email : jpott@keck.hawaii.edu Title : The enigma of GCIRS 3 Author(s): J.-U. Pott(1,2), A. Eckart(1), A. Glindemann(2), R. Sch\"odel(1), T. Viehmann(1) and M. Robberto(3) Institute: (1) I. Physikalisches Institut, University of Cologne, Z\"ulpicher Str. 77, D-50937 K\"oln, Germany, (2) European Southern Observatory (ESO), Karl-Schwarzschildstr. 2, D-85748 Garching bei M\"unchen, Germany, (3) Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA Paper : A&A in press Abstract: GCIRS~3 is the most prominent MIR-source in the central parsec of the Galaxy. NIR spectroscopy failed to solve the enigma of its nature. The properties and peculiarities of extreme individual objects of the central stellar cluster contribute to our knowledge of star and dust formation close to a supermassive black hole. We initiated an unprecedented interferometric experiment to understand the nature of GCIRS~3 and investigate its properties as spectroscopic and interferometric reference star at 10~$\mu$m. VLT/VISIR imaging separates a compact source from diffuse, surrounding emission. The VLTI/MIDI instrument was used to measure spectroscopically resolved visibility moduli at an angular resolution of $\sim$ 10~mas of that compact 10~$\mu$m source, still unresolved by a single VLT. Recent NIR/MIR photometry data were added to enable simple SED- and full radiative transfer-modeling of the data. The luminosity and size estimates show that IRS~3 is probably a cool carbon star enshrouded by a complex dust distribution. Blackbody temperatures were derived. The coinciding interpretation of single telescope and interferometric data confirm dust emission from several different spatial scales. The interferometric data resolve the inner rim of dust formation. Despite observed deep silicate absorption towards GCIRS~3 we favor a carbon rich circumstellar dust shell. The silicate absorption most probably takes place in the outer diffuse dust, which is mostly ignored by MIDI measurements, but well observable in complementary VLT/VISIR data. This indicates physically and chemically distinct conditions of the local dust, changing with the distance to GCIRS~3. We have demonstrated that optical long baseline interferometry at infrared wavelengths is an indispensable tool to investigate sources at the Galactic Center. Our findings suggest further studies of the composition of interstellar dust and the shape of the 10~$\mu$m silicate feature at this outstanding region. ------------------------------------------------------------------------ Email : martins@mpe.mpg.de Title : The most massive stars in the Arches cluster Author(s): F. Martins(1) and D.J. Hillier(2) and T. Paumard(3) and F. Eisenhauer(1) and T. Ott(1) and R. Genzel(1,4) Institute: (1) Max-Planck Instituet fuer extraterrestrische Physik, Postfach- 1312, D-85741, Garching bei Muenchen, Germany, (2) Department of Physics and Astronomy, University of Pittsburgh, 3941 O'Hara St., Pittsburgh, PA 15260, USA, (3) LESIA, Observatoire de Paris, CNRS, UPMC, Universite Paris Direrot, 5 Place Jules Janssen, F-92195, Meudon CEDEX, France, (4) Department of Physics, University of California, CA 94720, Berkeley, USA, Paper : A&A in press EPrint : 0711.0657 Abstract: We study a sample composed of 28 of the brightest stars in the Arches cluster. Our aim is to constrain their stellar and wind properties and to establish th eir nature and evolutionary status. We analyze K-band spectra obtained with the integral field spectrograph SINFONI on the VLT. Atmosphere models computed with the code CMFGEN are used to deri ve the effective temperatures, luminosities, stellar abundances, mass loss rates and wind terminal velocities. We find that the stars in our sample are either H-rich WN7-9 stars (WN7-9h) or supergiants, where two are classified as OIf^+. All stars are 2-4 Myr old. There is marginal evidence for a younger age among the most massive stars. The WN7-9h stars reach luminosities as high as 2 * 10^6 L_o, con sistent with initial masses of 120 Msun. They are still quite H-rich, but show both N enhancement and C depletion. They are thus identified as core H-bu rning objects showing products of the CNO equilibrium on their surface. Their progenitors are most likely supergiants of spectral types earlier than O4-6 and i nitial masses > 60 Msun. Their winds follow a well-defined modified wind momentum - luminosity relation (WLR): this is a strong indication that they are rad iatively driven. Stellar abundances tend to favor a slightly super-solar metallicity, at least for the lightest metals. We note, however, that the evolutionary models seem to under-predict the degree of N enrichment. ------------------------------------------------------------------------ Email : martins@mpe.mpg.de Title : On the nature of the fast moving star S2 in the Galactic Center Author(s): F. Martins(1), S. Gillessen(1), F. Eisenhauer(1), R. Genzel(1,2), T. Ott(1), S. Trippe(1) Institute: (1) Max Planck Institute of Extraterrestrial Physics, Postfach 1312, D-85741, Garching, Germany (2) Department of Physics, University of California, CA 94720, Berkeley, USA Paper : ApJL, accepted EPrint : 0711.3344 Abstract: We analyze the properties of the star S2 orbiting the supermassive black hole at the center of the Galaxy. A high quality SINFONI H and K band spectrum obtained from coadding 23.5 hours of observation between 2004 and 2007 reveals that S2 is an early B dwarf (B0-2.5V). Using model atmospheres, we constrain its stellar and wind properties. We show that S2 is a genuine massive star, and not the core of a stripped giant star as sometimes speculated to resolve the problem of star formation so close to the supermassive black hole. We give an upper limit on its mass loss rate, and show that it is He enriched, possibly because of the presence of a magnetic field. ------------------------------------------------------------------------ Email : rda@strw.leidenuniv.nl Title : Self-gravitating fragmentation of eccentric accretion disks Author(s): Richard D. Alexander(1), Philip J. Armitage(2), Jorge Cuadra, and Mitchell C. Begelman(2) Institute: JILA, 440 UCB, University of Colorado, Boulder, CO 80309-0440 (1) Leiden Observatory, Universiteit Leiden, Niels Bohrweg 2, 2300 RA, Leiden, the Netherlands (2) Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80309-0391 Paper : ApJ, Jan 2008, in press Web : http://www.strw.leidenuniv.nl/~rda/publications.html Abstract: We consider the effects of eccentricity on the fragmentation of gravitationally unstable accretion disks, using numerical hydrodynamics. We find that eccentricity does not affect the overall stability of the disk against fragmentation, but significantly alters the manner in which such fragments accrete gas. Variable tidal forces around an eccentric orbit slow the accretion process, and suppress the formation of weakly-bound clumps. The ``stellar'' mass function resulting from the fragmentation of an eccentric disk is found to have a significantly higher characteristic mass than that from a corresponding circular disk. We discuss our results in terms of the disk(s) of massive stars at 0.1pc from the Galactic Center, and find that the fragmentation of an eccentric accretion disk, due to gravitational instability, is a viable mechanism for the formation of these systems. ------------------------------------------------------------------------ (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 ========================================================================