======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 24, No. 8 Jun 22, 2006 Recently submitted papers: -------------------------- 1) Massive perturber-driven interactions of stars with a massive black hole (Perets et al., submitted) 2) The Rotation Measure and 3.5 mm Polarization of Sgr A* (Macquart et al., ApJ) 3) Chandra Observations of Galactic Center: High Energy Processes at Arcsecond Resolution (Wang, GC2006) 4) The Interplay between Star Formation and the Nuclear Environment of our Galaxy: Deep X-ray Observations of the Galactic Center Arches and Quintuplet Clusters (Wang et al., MNRAS) ------------------------------------------------------------------------ Email : hagai.perets@weizmann.ac.il Title : Massive perturber-driven interactions of stars with a massive black hole Author(s): Hagai B. Perets(1), Clovis Hopman(1,2) and Tal Alexander(1) Institute: (1) Faculty of Physics, Weizmann Institute of Science, POB 26, Rehovot 76100, Israel (2) Leiden Observatory, P.O. box 9513, NL-2300 RA Leiden Paper : submitted to ApJ EPrint : astro-ph/0606443 Abstract: We study the role of massive perturbers (MPs) in deflecting stars and binaries to almost radial (``loss-cone'') orbits, where they pass near the central massive black hole (MBH), interact with it at periapse q, and are ultimately destroyed. MPs dominate dynamical relaxation when the ratio of the 2nd moments of the MP and star mass distributions, mu _2\!\equiv\!<=ft.N_p<=ft< ngle M_p^2\right\rangle \right/N_\star<=ft< ngle M_\star^2\right\rangle , satisfies mu _2\!\gg\!1. The observed MPs in the nucleus of the Galaxy (giant molecular clouds and stellar clusters), and plausibly in late type galaxies generally, have 10^2\!<=sssim\! mu _2\!<=sssim\!10^5. MPs thus shorten the relaxation timescale by 10^2-5 relative to 2-body relaxation by stars alone. We show this increases by 10^1-3 the rate of large-q interactions with the MBH, where loss-cone refilling by stellar 2-body relaxation is inefficient. We extend the Fokker-Planck loss-cone formalism to approximately account for relaxation by rare encounters with MPs. We show that binary-MBH exchanges driven by MPs may explain the origin of the young main sequence B stars that are observed very near the Galactic MBH, and may increase by orders of magnitude the ejection rate of hyper-velocity stars. We suggest that MP-driven relaxation plays an important role in the capture of stars on very tight orbits around the MBH, leading to their tidal orbital decay and disruption or to inspiral through the emission of gravitational waves from zero-eccentricity orbits. We show that loss-cone refilling by MPs leads to rapid orbital decay and coalescence of binary MBHs, thereby solving the stalling problem. ------------------------------------------------------------------------ Email : jpm@astro.caltech.edu Title : The Rotation Measure and 3.5 mm Polarization of Sgr A* Author(s): Jean-Pierre Macquart(1), Geoffrey C. Bower(2), Melvyn C.H. Wright(2), Donald C. Backer(2), Heino Falcke(3) Institute: (1) NRAO, P.O. Box 0, Socorro NM 87801, U.S.A. (Jansky Fellow) (2) Astronomy Department and Radio Astronomy Laboratory, University of California, Berkeley, Berkeley, CA 94720, U.S.A. (3) ASTRON, Postbus 2, 7990 AA Dwingeloo, The Netherlands and Department of Astrophysics, Radboud Universiteit Nijmegen, Postbus 9010, 6500 GL Nijmegen, The Netherlands Paper : ApJ Lett, Jun 2006, in press EPrint : astro-ph/0606381 Abstract: We report the detection of variable linear polarization from Sgr A* at a wavelength of 3.5 mm, the longest wavelength yet at which a detection has been made. The mean polarization is 2.1 +/- 0.1% at a position angle of 16 +/- 2^o with rms scatters of 0.4% and 9^o over the five epochs. We also detect polarization variability on a timescale of days. Combined with previous detections over the range 150-400 GHz (750-2000 micron ), the average polarization position angles are all found to be consistent with a rotation measure of -4.4 +/- 0.3 * 10^5 rad m^-2. This implies that the Faraday rotation occurs external to the polarized source at all wavelengths. This implies an accretion rate 0.2 - 4 * 10^-8 M_o yr^-1 for the accretion density profiles expected of ADAF, jet and CDAF models and assuming that the region at which electrons in the accretion flow become relativistic is within 10 R_S. The inferred accretion rate is inconsistent with ADAF/Bondi accretion=2E The stability of the mean polarization position angle between disparate polarization observations over the frequency range limits fluctuations in the accretion rate to less than 5%. The flat frequency dependence of the inter-day polarization position angle variations also makes them difficult to attribute to rotation measure fluctuations, and suggests that both the magnitude and position angle variations are intrinsic to the emission. ------------------------------------------------------------------------ Email : wqd@astro.umass.edu Title : Chandra Observations of Galactic Center: High Energy Processes at Arcsecond Resolution Author(s): Q. Daniel Wang^1 Institute: (1) Department of Astronomy, University of Massachusetts, Amherst, MA 01003 Paper : GC2006 Invited Talk EPrint : astro-ph/0606414 Web : http://ww.astro.umass.edu/~wqd/papers/arches/gc2006.pdf Abstract: About 2 million seconds of \chandra observing time have been devoted to the Galactic center (GC), including large-scale surveys and deep pointings. These observations have led to the detection of about 4000 discrete X-ray sources and the mapping of diffuse X-ray emission in various energy bands. In this review, I first summarize general results from recent studies and then present close-up views of the three massive star clusters (Arches, Quintuplet, and GC) and their interplay with the Galactic nuclear environment. ------------------------------------------------------------------------ Email : wqd@astro.umass.edu Title : The Interplay between Star Formation and the Nuclear Environment of our Galaxy: Deep X-ray Observations of the Galactic Center Arches and Quintuplet Clusters Author(s): Q. Daniel Wang(1), Hui Dong(1) and Cornelia Lang(2) Institute: (1) Department of Astronomy, B619E-LGRT, University of Massachusetts, Amherst, MA 01003 (2) Department of Physics and Astronomy, University of Iowa, Iowa City, IA 52245 Paper : MNRAS in press EPrint : astro-ph/0606282 Web : http://ww.astro.umass.edu/~wqd/papers/arches/ Abstract: The Galactic center (GC) provides a unique laboratory for a detailed examination of the interplay between massive star formation and the nuclear environment of our Galaxy. Here, we present an 100 ks Chandra ACIS observation of the Arches and Quintuplet star clusters. We also report on a complementary mapping of the dense molecular gas near the Arches cluster made with the Owens Valley Millimeter Array. We present a catalog of 244 point-like X-ray sources detected in the observation. Their number-flux relation indicates an over-population of relatively bright X-ray sources, which are apparently associated with the clusters. The sources in the core of the Arches and Quintuplet clusters are most likely extreme colliding wind massive star binaries. The diffuse X-ray emission from the core of the Arches cluster has a spectrum showing a 6.7-keV emission line and a surface intensity profile declining steeply with radius, indicating an origin in a cluster wind. In the outer regions near the Arches cluster, the overall diffuse X-ray enhancement demonstrates a bow shock morphology and is prominent in the Fe K alpha 6.4-keV line emission with an equivalent width of 1.4 keV. Much of this enhancement may result from an ongoing collision between the cluster and the adjacent molecular cloud, which have a relative velocity > 120 km s^-1. The older and less compact Quintuplet cluster contains much weaker X-ray sources and diffuse emission, probably originating from low-mass stellar objects as well as a cluster wind. However, the overall population of these objects, constrained by the observed total diffuse X-ray luminosities, is substantially smaller than expected for both clusters, if they have normal Miller \& Scalo initial mass functions. This deficiency of low-mass objects may be a manifestation of the unique star formation environment of the Galactic center, where high-velocity cloud-cloud and cloud-cluster collisions are frequent. ------------------------------------------------------------------------ (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 ========================================================================