======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 28, No. 3 Mar 20, 2008 Recently submitted papers: -------------------------- 1) Odin observations of the Galactic centre in the 118-GHz band. Upper limit to the O_2 abundance (Sandqvist et al., A&A) 2) A Wide-area VLA Continuum Survey near the Galactic Center at 6 and 20 cm Wavelengths (Law et al., ApJS) 3) Massive Star Formation in the Galactic Center (al., ) 4) Time Variability of the Neutral Iron Lines from the Sgr B2 Region and its Implication of a Past Outburst of Sgr A^* (Inui et al., PASJ) 5) On a Source of Systematic Error in Absolute Measurement of Galactocentric Distance from Solving for the Stellar Orbit Around Sgr A* (Nikiforov, Dynamics) 6) Radial Velocities of Stars in the Galactic Center (Zhu et al., ApJ) ------------------------------------------------------------------------ Email : aage@astro.su.se Title : Odin observations of the Galactic centre in the 118-GHz band. Upper limit to the O_2 abundance Author(s): Aa. Sandqvist(1), B. Larsson(1), \AA . Hjalmarson(2), P. Bergman(2), P. Bernath(3), U. Frisk(4), M. Olberg(2), L. Pagani(5), L.M. Ziurys(6) Institute: (1) Stockholm Observatory, AlbaNova University Center, SE-106 91 Stockholm, Sweden (2) Onsala Space Observatory, SE-439 92 Onsala, Sweden (3) Department of Chemistry, University of York, Heslington, York, YO10 5DD, UK (4) Swedish Space Corporation, P. O. Box 4207, SE-171 04 Solna, Sweden (5) LERMA \& UMR8112 du CNRS, Observatoire de Paris, 61, FR-75014 Paris, France (6) Arizona Radio Observatory, Steward Observatory, University of Arizona, Tucson, AZ 85721, USA Paper : A&A, in press EPrint : 0803.1726 Abstract: The Odin satellite has been used to search for the 118.75-GHz line of molecular oxygen (O_2) in the Galactic centre. Odin observations were performed towards the Sgr A^* circumnuclear disk (CND), and the Sgr A +20 km s^-1 and +50 km s^-1 molecular clouds using the position-switching mode. Supplementary ground-based observations were carried out in the 2-mm band using the ARO Kitt Peak 12-m telescope to examine suspected SiC features. A strong emission line was found at 118.27 GHz, attributable to the J=13-12 HC_3N line. Upper limits are presented for the 118.75-GHz O_2 (1_1-1_0) ground transition line and for the 118.11-GHz ^3\Pi_2, J=3-2 ground state SiC line at the Galactic centre. Upper limits are also presented for the 487-GHz O_2 line in the Sgr A +50 km s^-1 cloud and for the 157-GHz, J=4-3, SiC line in the Sgr A +20 and +50 km s^-1 clouds, as well as the CND. The CH_3OH line complex at 157.2 - 157.3 GHz has been detected in the +20 and +50 km s^-1 clouds but not towards Sgr A^*/CND. A 3 sigma upper limit for the fractional abundance ratio of [O_2]/[H_2] is found to be X(O_2) <= 1.2 * 10^-7 towards the Sgr A molecular belt region. ------------------------------------------------------------------------ Email : caseyjlaw@gmail.com Title : A Wide-area VLA Continuum Survey near the Galactic Center at 6 and 20 cm Wavelengths Author(s): C. J. Law(1,2), F. Yusef-Zadeh(2), W. D. Cotton(3) Institute: (1) Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, USA (2) stronomical Institute ``Anton Pannekoek'', University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands (3) National Radio Astronomy Observatory, Charlottesville, VA 22903, USA Paper : ApJS, August 2008 , in press EPrint : 0803.1412 Abstract: We describe the results of a mJy-sensitivity, VLA survey of roughly 1 square degree near the Galactic center at 6 and 20 cm. Catalogs of compact and filamentary structures are given and compared to previous surveys of the region. Eight of the unusual, nonthermal radio filaments are detected in 6 cm polarized emission; three of these are the first such detections, confirming their nonthermal nature. This survey found emission from a filament at (l,b)=(359.1,0.75), or a projected distance from Sgr A* of 200 pc, greatly extending the latitude range observed with such features. There is also new evidence for spatial gradients in the 6/20 cm spectral indices of some filaments and we discuss models for these gradients. In studying compact sources, the combination of spectral index and polarization information allows us to identify pulsar candidates and compact \hii regions in the survey. There is also some evidence that the flux measurements of compact sources may be affected by electron scattering from the interstellar medium in the central few hundred parsecs of the Galaxy. ------------------------------------------------------------------------ Email : cmtpci@cis.rit.edu Title : Massive Star Formation in the Galactic Center Author(s): D. F. Figer et al. Institute: Rochester Institute of Technology, Rochester, NY, USA EPrint : 0803.1619 Web : http://arxiv.org/abs/0803.1619 Abstract: The Galactic center is a hotbed of star formation activity, containing the most massive star formation site and three of the most massive young star clusters in the Galaxy. Given such a rich environment, it contains more stars with initial masses above 100 \Msun than anywhere else in the Galaxy. This review concerns the young stellar population in the Galactic center, as it relates to massive star formation in the region. The sample includes stars in the three massive stellar clusters, the population of younger stars in the present sites of star formation, the stars surrounding the central black hole, and the bulk of the stars in the field population. The fossil record in the Galactic center suggests that the recently formed massive stars there are present-day examples of similar populations that must have been formed through star formation episodes stretching back to the time period when the Galaxy was forming. ------------------------------------------------------------------------ Email : inuit@cr.scphys.kyoto-u.ac.jp Title : Time Variability of the Neutral Iron Lines from the Sgr B2 Region and its Implication of a Past Outburst of Sgr A^* Author(s): Tatsuya Inui, Katsuji Koyama, Hironori Matsumoto and Takeshi Go Tsuru Institute: (1) Department of Physics, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502 Paper : PASJ, 2008, accepted Abstract: We investigate long-term X-ray behaviors from the Sgr B2 complex using archival data of the X-ray satellites Suzaku, XMM-Newton, Chandra and ASCA. The observed region of the Sgr B2 complex includes two prominent spots in the Fe\emissiontypeI K- alpha line at 6.40 keV, a giant molecular cloud M 0.66-0.02 known as the ``Sgr B2 cloud'' and an unusual X-ray source G 0.570-0.018. Although these 6.40 keV spots have spatial extensions of a few pc scale, the morphology and flux of the 6.40 keV line has been time variable for 10 years, in contrast to the constant flux of the Fe\emissiontypeXXV-K alpha line at 6.67 keV in the Galactic diffuse X-ray emission. This time variation is mostly due to M 0.66-0.02; the 6.40 keV line flux declined in 2001 and decreased to 60% in the time span 1994-2005. The other spot G 0.570-0.018 is found to be conspicuous only in the Chandra observation in 2000. From the long-term time variability ( 10 years) of the Sgr B2 complex, we infer that the Galactic Center black hole Sgr A^* was X-ray bright in the past 300 year and exhibited a time variability with a period of a few years. ------------------------------------------------------------------------ Email : nii@dyna.astro.spbu.ru Title : On a Source of Systematic Error in Absolute Measurement of Galactocentric Distance from Solving for the Stellar Orbit Around Sgr A* Author(s): Igor' I. Nikiforov Institute: (1) Sobolev Astronomical Institute, St. Petersburg State University, Universitetskij pr. 28, Staryj Peterhof, St. Petersburg 198504, Russia, nii@astro.spbu.ru Paper : Dynamics of Galaxies, Proceedings of the EPrint : 0803.0825 Abstract: Eisenhauer et al. (2003, 2005) derived absolute (geometrical) estimates of the distance to the center of the Galaxy, R_0, from the star S2 orbit around Sgr A* on the assumption that the intrinsic velocity of Sgr A* is negligible. This assumption produces the source of systematic error in R_0 value owing to a probable motion of Sgr A* relative to the accepted velocity reference system which is arbitrary to some extent. Eisenhauer et al. justify neglecting all three spatial velocity components of Sgr A* mainly by low limits of Sgr A*'s proper motion of 20-60 km/s. In this brief paper, a simple analysis in the context of the Keplerian dynamics was used to demonstrate that neglect of even low (perhaps, formal) radial velocity of Sgr A* leads to a substantial systematic error in R_0: the same limits of 20-60 km/s result in R_0 errors of 1.3-5.6%, i.e., (0.1-0.45)* (R_0/8) kpc, for current S2 velocities. Similar values for Sgr A*'s tangential motion can multiply this systematic error in the case of S2 orbit by factor 1.5-1.9 in the limiting cases. ------------------------------------------------------------------------ Email : qxzpci@cis.rit.edu Title : Radial Velocities of Stars in the Galactic Center Author(s): Qingfeng Zhu(1), Rolf P. Kudritzki(2), Donald F. Figer(1), Francisco Najarro(3), David Merritt(4) Institute: (1) Chester F. Carlson Center for Imaging Science, Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester, NY 14623-5604 (2) Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822 (3) Insituto de Estructura de la Materia, Consejo Superior de Investigaciones Cientificas, Calle Serrano 121, 28006, Madrid, Spain (4) Department of Physics and Center for Computational Relativity and Gravitation, 85 Lomb Memorial Drive, Rochester, NY 14623-5604 Paper : ApJ, July 2008, in press EPrint : 0803.1826 Abstract: We present results from K band slit scan observations of a 20''x20'' region of the Galactic center (GC) in two separate epochs more than five years apart. The high resolution (R>=14,000) observations allow the most accurate radial velocity and acceleration measurements of the stars in the central parsec of the Galaxy. Detected stars can be divided into three groups based on the CO absorption band heads at 2.2935 micron and the He I lines at 2.0581 micron and 2.112, 2.113 micron : cool, narrow-line hot and broad-line hot. The radial velocities of the cool, late-type stars have approximately a symmetrical distribution with its center at -7.8+/-10.3 km s^-1 and a standard deviation 113.7+/-10.3 km s^-1. Although our statistics are dominated by the brightest stars, we estimate a central black hole mass of 3.9+/-1.1*10^6 solar masses, consistent with current estimates from complete orbits of individual stars. Our surface density profile and the velocity dispersion of the late type stars support the existence of a low density region at the Galactic center suggested by earlier observations. Many hot, early-type stars show radial velocity changes higher than maximum values allowed by pure circular orbital motions around a central massive object, suggesting that the motions of these stars greatly deviate from circular orbital motions around the Galactic center. The correlation between the radial velocities of the early type He I stars and their declination offsets from Sagittarius A* suggests that a systematic rotation is present for the early-type population. No figure rotation around the Galactic center for the late type stars is supported by the new observations. ------------------------------------------------------------------------ (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 ========================================================================