======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@mpifr-bonn.mpg.de http://www.mpifr-bonn.mpg.de/gcnews ======================================================================== Vol. 14, No. 8 Dec 20, 2001 The response to the next Galactic Center Workshop in Hawaii (http://www.gemini.edu/science/gc_conf/) is very strong. We had 60 people within the first 2 days filling out the survey form at http://www.mpifr-bonn.mpg.de/gcnews/survey.shtml indicating their interest. If you think about attending and have not filled out the form, please do so soon so that we can reserve enough rooms. Recently submitted papers: -------------------------- 1) Stellar Orbits Near Sagittarius A* (Eckart et al., MNRAS) 2) A Jet-ADAF model for Sgr A* (Yuan et al., A&A) ------------------------------------------------------------------------ Email : eckart@ph1.uni-koeln.de Title : Stellar Orbits Near Sagittarius A* Author(s): A. Eckart(1), R. Genzel(2), T. Ott(2), R. Schoedel(2) Institute: (1) I.Physikalisches Institut, Universitaet zu Koeln, Zuelpicher Str.77, 50937 Koeln, Germany (2) Max-Planck-Institut fuer extraterrestrische Physik (MPE), D-85740 Garching, Germany Paper : MNRAS, in press Abstract: The SHARP/NTT stellar proper motion data now cover an interval from 1992 to 2000 and allow us to determine orbital accelerations for some of the most central stars. We confirm the stellar acceleration measurements obtained by Ghez et al. (2000) with NIRC at the Keck telescope. Our analysis differs in 3 main points from that of Ghez et al.: 1) We combine the high precision but shorter time scale NIRC/Keck data with the lower precision but longer time scale SHARP/NTT data set; 2) We statistically correct the observed accelerations for geometrical projection effects; 3) We exclude star S8 from the analysis of the amount and position of the central mass. From the combined SHARP/NTT and NIRC/Keck data sets we show that the stars S2, and most likely S1 and S8 as well, are on bound, fairly inclined (60^o < i < 80^o), and eccentric (0.4< e < 0.95) orbits around a central dark mass. The combination of both data sets results in a position of this central mass of 48^+54_-24 mas E and 18^+42_-61 mas S of the nominal radio position of Sgr A*. The mean statistically corrected enclosed mass derived from accelerations is M_acc=(5 +/- 3) * 10^6M_o with current radial separations of S1 and S2 from SgrA* of about 8-10 mpc. This enclosed mass estimate is derived from individual stellar orbits as close to the massive black hole at the center of the Milky Way as currently possible. Although the uncertainties are large this estimate is fully consistent with the enclosed mass range of (2.6-3.3)*10^6M_o derived by Genzel et al. (2000) from radial and/or proper motion velocities of a homogenized sample of sources. Star S8 was excluded from the analysis, since for the current proper motion velocity and radial separation from the center we find that the measured acceleration requires orbital motion around a compact object with a mass in excess of 3*10^6M_o. The data suggest that this star either was or is subject to a close interaction with a different object or that its position measurements are influenced by the emission of a different cluster star. Therefore, we base the analysis of the enclosed mass solely on the available data for S1 and S2. We also discuss two late type stars with projected separations from Sgr A* of about 0.5'' and 1''. In addition to proper motions these stars have known radial velocities. Orbit calculations indicate that those stars are very likely at larger physical distances from the center and part of the larger scale central stellar cluster with a core radius of approximately 0.3 pc. ------------------------------------------------------------------------ Email : hfalcke@mpifr-bonn.mpg.de Title : A Jet-ADAF model for Sgr A^* Author(s): Feng Yuan (1,2), Sera Markoff (1), Heino Falcke (1) Institute: (1) Max-Planck-Institut fuer Radioastronomie, Auf dem Huegel 69, D-53121 Bonn, Germany (2) Also at CAS-PKU Joint Beijing Astrophysical Centre, Beijing 100871, China; and Astronomical and Astrophysical Center of East China, Nanjing University, Nanjing 210093, China. Paper : A&A, in press EPrint : astro-ph/0112464 Abstract: The recent Chandra observation of the radio source at the center of our Galaxy, Sgr A*, puts new constraints on its theoretical models. The spectrum is very soft, and the source is rapidly variable. We consider different models to explain the observations. We find that the features of the x-ray spectrum can be marginally explained with an advection-dominated accretion flow (ADAF) model while it does not well fit the radio spectrum. An ADAF with strong winds (ADIOS) model is not favored if we assume that the wind does not radiate. Alternatively, we propose a coupled jet plus accretion disk model to explain the observations for Sgr A*. The accretion flow is described as an ADAF fed by Bondi-Hoyle accretion of hot plasma in the Galactic Center region. A small fraction of the accretion flow is ejected near the black hole, forming a jet after passing through a shock. As a result, the electron temperature increases to 2*10^11K, which is about 10 times higher than the highest temperature attained in the ADAF. The model is self-consistent since the main jet parameters are determined by the underlying accretion disk at the inner edge. The emergent spectrum of Sgr A^* is the sum of the emission from jet and underlying ADAF. The very strong Comptonization of synchrotron emission from the jet dominates the bremsstrahlung from the ADAF, therefore, a very short variability timescale is expected and the predicted X-ray slope and the radio spectrum is in very good agreement with the observations. ------------------------------------------------------------------------ (Older versions of the Newsflash can be found at the gcnews web-page) ======================================================================== Edited by Angela Cotera Heino Falcke & Sera Markoff (cotera@as.arizona.edu) (hfalcke,smarkoff@mpifr-bonn.mpg.de) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - For Abstract submission please send the (La)Tex file of your paper to gcnews@mpifr-bonn.mpg.de ========================================================================