======================================================================== 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. 8 Sep 27, 2007 Recently submitted papers: -------------------------- 1) Star formation in accretion discs :, from the Galactic Center to Active Galactic Nuclei (Collin & Zahn, A&A) 2) Implementing Few-Body Algorithmic Regularization with Post-Newtonian Terms (Mikkola & Merritt, arXiv:0709.3367) 3) The Galactic center, the largest oxigen bearing organic molecule repository (Requena-Torres et al., ApJ) 4) Point Sources from a Spitzer IRAC Survey of the Galactic Center (Ramirez et al., ApJS) 5) Flares from spiral waves by lensing and time-delay amplification? (Karas et al., Proceedings) ------------------------------------------------------------------------ Email : Suzy.Collin@obspm.fr Title : Star formation in accretion discs :, from the Galactic Center to Active Galactic Nuclei Author(s): Suzy Collin^1, Jean-Paul Zahn^1 Institute: (1) LUTH Observatoire de Paris, Section de Meudon, 92195F Meudon, France Paper : A&A, accepted EPrint : 0709.3772 Abstract: Keplerian accretion discs around massive black holes (MBHs) are gravitationally unstable beyond a few hundredths of parsec and should collapse to form stars. It was indeed shown recently that an accretion/star formation episode took place a few millions years ago in the Galactic Center (GC). This raises the question of how the disc can survive in AGN and quasars and continue to transport matter towards the black hole. We study the accretion/star formation process in quasars and AGN, with one aim in mind, to show that a spectrum similar to the observed one can be produced by the disc.We compute models of stationary accretion discs, both continuous and clumpy. Continuous discs must be maintained in a state of marginal stability so that the rate of star formation remains modest and the disc is not immediately destroyed. The disc then requires an additional heating and an additional transport of angular momentum. In clumpy discs the momentum transport is provided by cloud interactions. Non-viscous heating can be provided by stellar illumination, but in the case of continuous discs, even momentum transport by supernovae is insufficient to sustain a marginal state, except at the very periphery of the disc. In clumpy discs it is possible to account for the required accretion rate through interactions between clouds, but this model is unsatisfactory as its parameters are tightly constrained without any physical justification. So finally one must appeal to non-stationary discs with intermittent accretion episodes like those that occurred in the GC, but such a model is probably not applicable to luminous high redshift quasars neither to radio-loud quasars. ------------------------------------------------------------------------ Email : merritt@astro.rit.edu Title : Implementing Few-Body Algorithmic Regularization with Post-Newtonian Terms Author(s): Seppo Mikkola David Merritt Institute: (1) Tuorla Observatory, University of Turku, Vaeisaelaentie 20, Piikkioe, Finland (2) Center for Computational Relativity and Gravitation and Department of Physics, Rochester Institute of Technology, Rochester, NY 14623, USA Paper : arXiv:0709.3367 Web : http://ccrg.rit.edu/Research/Publications.php Abstract: We discuss the implementation of a new regular algorithm for simulation of the gravitational few-body problem. The algorithm uses components from earlier methods, including the chain structure, the logarithmic Hamiltonian, and the time-transformed leapfrog. The code can be used for the normal N-body problem, as well as for problems with softened potentials and/or with velocity-dependent external perturbations, including post-Newtonian terms, which we include up to order PN2.5. Arbitarily extreme mass ratios are allowed. Coordinate transformations are not used and thus the algorithm is somewhat simpler than many earlier regularized schemes. We present the results of performance tests, then use our algorithm to integrate the orbits of the S stars around the Milky Way supermassive black hole for one million years, including PN2.5 terms and an intermediate-mass black hole. The three S stars with shortest periods are observed to escape from the system after a few hundred thousand years. ------------------------------------------------------------------------ Email : requena@damir.iem.csic.es Title : The Galactic center, the largest oxigen bearing organic molecule repository Author(s): Requena-Torres, M.A., Martin-Pintado, J., Martin, S., Morris, M. R. Institute: (1) Departamento de Astrofisica Molecular e Infrarroja-Instituto de Estructura de la Materia-CSIC, C Serrano 121, E-28006 Madrid, Spain (2) Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 78 Cambridge, MA 02138, USA (3) Department of Physics \& Astronomy, University of California, Los Angeles, CA 90095-1547, USA Paper : ApJ, Jan 2008, in press EPrint : 0709.0542 Abstract: We present the first detection of complex aldehydes and isomers in three typical molecular clouds located within 200 pc of the center of our Galaxy. We find very large abundances of these complex organic molecules (COMs) in the central molecular zone (CMZ), which we attribute to the ejection of COMs from grain mantles by shocks. The relative abundances of the different COMs with respect to that of CH_3OH are strikingly similar for the three sources, located in very different environments in the CMZ. The similar relative abundances point toward a unique grain mantle composition in the CMZ. Studying the Galactic center clouds and objects in the Galactic disk having large abundances of COMs, we find that more saturated molecules are more abundant than the non-saturated ones. We also find differences between the relative abundance between COMs in the CMZ and the Galactic disk, suggesting different chemical histories of the grain mantles between the two regions in the Galaxy for the complex aldehydes. Different possibilities for the grain chemistry on the icy mantles in the GC clouds are briefly discussed. Cosmic rays can play an important role in the grain chemistry. With these new detections, the molecular clouds in the Galactic center appear to be one of the best laboratories for studying the formation of COMs in the Galaxy. ------------------------------------------------------------------------ Email : solange@ipac.caltech.edu Title : Point Sources from a Spitzer IRAC Survey of the Galactic Center Author(s): Solange V. Ramirez Richard G. Arendt Kris Sellgren Susan R. Stolovy Angela Cotera Howard A. Smith Farhad Yusef-Zadeh Institute: (1) IPAC/Caltech, Pasadena, CA 91125 (2) CRESST/UMBC/GSFC, Code 665, Greenbelt, MD 20771 (3) The Ohio State University, Columbus, OH 43210 (4) Spitzer Science Center, Caltech, Pasadena, CA 91125 (5) SETI Institute, Mountain View, CA 94043 (6) Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138 (7) Northwestern University, Evanston, IL 60208 Paper : ApJS, 2008, in press EPrint : 0709.3113 Abstract: We have obtained Spitzer/IRAC observations of the central 2.0 DEGree * 1.4 DEGree ( 280 * 200 pc) of the Galaxy at 3.6 micron -8.0 micron . A point source catalog of 1,065,565 objects is presented. The catalog includes magnitudes for the point sources at 3.6, 4.5, 5.8, and 8.0 micron , as well as JHK_s photometry from 2MASS. The point source catalog is confusion limited with average limits of 12.4, 12.1, 11.7, and 11.2 magnitudes for [3.6], [4.5], [5.8], and [8.0], respectively. We find that the confusion limits are spatially variable because of stellar surface density, background surface brightness level, and extinction variations across the survey region. The overall distribution of point source density with Galactic latitude and longitude is essentially constant, but structure does appear when sources of different magnitude ranges are selected. Bright stars show a steep decreasing gradient with Galactic latitude, and a slow decreasing gradient with Galactic longitude, with a peak at the position of the Galactic center. From IRAC color-magnitude and color-color diagrams, we conclude that most of the point sources in our catalog have IRAC magnitudes and colors characteristic of red giant and AGB stars. ------------------------------------------------------------------------ Email : vladimir.karas@cuni.cz Title : Flares from spiral waves by lensing and time-delay amplification? Author(s): Vladimir Karas, Michal Dovciak, Andreas Eckart and Leonhard Meyer Paper : Proceedings "Ragtime 9" workshop, in preparation Abstract: Episodically accreting black holes are thought to produce flares when a chunk of particles is accelerated to high velocity near the black hole horizon. This also seems to be the case of Sagittarius A* in the Galactic Center, where the broad-band radiation is produced, likely via the synchrotron self-Compton mechanism. It has been proposed that strong-field gravitational lensing magnifies the flares. The effect of lensing is generally weak and requires a fine-tuned geometrical arrangement, which occurs with only a low probability. However, there are several aspects that make Sagittarius A* a promising target to reveal strong gravity effects. Unlike type II (obscured) active galaxies, chances are that a flare is detected at high inclination, which would be favourable for lensing. Time delays can then significantly influence the observed flare duration and the form of light-curve profiles. Here we discuss an idea that the impact of lensing amplification should be considerably enhanced when the shape of the flaring clump is appropriately elongated in the form of a spiral wave or a narrow filament, rather than a simple (circular) spot which we employed previously within the phenomenological `orbiting spot model'. By parameterizing the emission region in terms of the spiral shape and contrast, we are able to extend the spot model to more complicated sources. In the case of spirals, we notice a possibility that more photons reach a distant observer at the same moment because of interplay between lensing and light-travel time. The effect is not symmetrical with respect to leading versus trailing spirals, so in principle the source geometry can be constrained. In spite of this, the spot model seems to provide entirely adequate framework to study the currently available data. ------------------------------------------------------------------------ (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 ========================================================================