======================================================================== 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. 1 Mar 9, 2006 Recently submitted papers: -------------------------- 1) Stochastic Acceleration in the Galactic Center HESS Source (Liu et al., ApJ) 2) Testing the Stochastic Acceleration Model for Flares in Sagittarius A* (Liu et al., ApJL) 3) The Two Young Star Disks in the Central Parsec of the Galaxy: Properties, Dynamics and Formation (Paumard et al., ApJ) 4) Understanding the Stellar Initial Mass Function (Larson, Revista) 5) Folded Fields as the Source of Extreme Radio-Wave Scattering in the Galactic Center (Sridhar, APJL) ------------------------------------------------------------------------ Email : liusm@lanl.gov Title : Stochastic Acceleration in the Galactic Center HESS Source Author(s): Siming Liu(1), Fulvio Melia(2,3), Vahe Petrosian(4) and Marco Fatuzzo(5) Institute: (1) Los Alamos National Laboratory, Los Alamos, NM, 87545 (2) Physics Department and Steward Observatory, The University of Arizona, Tucson, AZ 85721 (3) Sir Thomas Lyle Fellow and Miegunyah Fellow. (4) Center for Space Science and Astrophysics, Department of Applied Physics, Stanford University, Stanford, CA 94305 (5) Physics Department, Xavier University, Cincinnati, OH 45207 Paper : ApJ, Submitted EPrint : astro-ph/0603137 Abstract: Stochastic acceleration of charged particles interacting resonantly with a turbulent magnetic field in a small accretion torus appears to be the likely mechanism responsible for much of Sagittarius A*'s mm and shorter wavelength spectrum. The longer wavelength radiation is produced at larger radii by electrons either diffusing from smaller scales or accelerated in situ. An important prediction of this model is the ejection of a significant flux of relativistic protons from a magnetic-field-dominated acceleration site into the wind-shocked medium surrounding the black hole. Recently, several air \v Cerenkov telescopes, notably HESS, have detected TeV emission from within 1' of the Galactic Center, with characteristics hinting at a pp-induced pion decay process for the gamma -ray emission. Given (i) that we now know the size of this acceleration region, where Sagittarius A*'s 7-mm wavelength emission originates, and (ii) that we can now map the wind-injected ISM within 3 pc of the nucleus using the diffuse X- rays detected with Chandra, it is feasible to test the idea that protons accelerated within 20 Schwarzschild radii of the black hole produce the TeV emission farther out. We show that the diffusion length of these particles away from their source guarantees a majority of TeV protons scattering at least once within 3 pc of Sagittarius A*, and we demonstrate that the proton power ( 10^37 ergs s^-1) produced in concert with the 7-mm radio emission matches the TeV luminosity well. The particle cascade generated by the pp scatterings produces GeV gamma -rays from \pi^0 decays, and bremsstrahlung, inverse Compton, and synchrotron emission at longer wavelengths from secondary particles. We compare these with current measurements and demonstrate that GLAST will detect this source during its one-year all-sky survey. This model explains why the TeV source is unresolved, yet does not vary on a time scale of a year or less, and it also accounts for the high-energy emission while retaining consistency with Sgr A*'s well- studied cm and mm characteristics. ------------------------------------------------------------------------ Email : liusm@lanl.gov Title : Testing the Stochastic Acceleration Model for Flares in Sagittarius A* Author(s): Siming Liu(1), Vahe Petrosian(2), Fulvio Melia(3,4), and Christopher L. Fryer(1,5) Institute: (1) Los Alamos National Laboratory, Los Alamos, NM 87545 (2) Center for Space Science and Astrophysics, Department of Physics and Applied Physics, Stanford University, Stanford, CA 94305 (3) Physics Department and Steward Observatory, The University of Arizona, Tucson, AZ 85721 (4) Sir Thomas Lyle Fellow and Miegunyah Fellow. (5) Physics Department, The University of Arizona, Tucson, AZ 85721 Paper : ApJL, submitted EPrint : astro-ph/0603136 Abstract: The near-IR and X-ray flares in Sagittarius A* are believed to be produced by relativistic electrons via synchrotron and synchrotron self-Comptonization (SSC), respectively. These electrons are likely energized by turbulent plasma waves through second order Fermi acceleration that, in combination with the radiative cooling processes, produces a relativistic Maxwellian distribution in the steady state. This model has four principal parameters, namely the magnetic field B, the electron density n, their ``temperature'' gamma _c m_e c^2, and the size of the flare region R. In the context of stochastic acceleration by plasma waves, the quantities R n^1/2 B and gamma _c R n should remain nearly constant in time. Therefore, simultaneous spectroscopic observations in the NIR and X-ray bands can readily test the model, which, if proven to be valid, may be used to determine the evolution of the plasma properties during an eruptive event with spectroscopic observations in either band or simultaneous flux density measurements in both bands. The formulation we develop here may also be applicable to other sources radiating via thermal synchrotron and SSC processes. ------------------------------------------------------------------------ Email : paumard@iap.fr Title : The Two Young Star Disks in the Central Parsec of the Galaxy: Properties, Dynamics and Formation Author(s): T. Paumard(1), R. Genzel(1,2), F. Martins(1), S. Nayakshin(3,4), A. M. Beloborodov(5,6), Y. Levin(7,8), S. Trippe(1), F. Eisenhauer(1), T. Ott(1), S. Gillessen(1), R. Abuter(1), J. Cuadra(3), T. Alexander(9,10), A. Sternberg(11) Institute: (1) Max-Planck Institut fuer extraterrestrische Physik (MPE), Giessenbachstrasse, 85748 Garching, Germany (2) Department of Physics, University of California, 366 LeConte Hall, Berkeley, CA 94720, USA (3) Max-Planck Institut fuer Astrophysik (MPA), Karl-Schwarzschild-Str. 1, 85741 Garching, Germany (4) Theoretical Astrophysics Group, Department of Physics \& Astronomy, University of Leicester, Leicester, LE1 7RH, United Kingdom (5) Physics Department and Columbia Astrophysics Laboratory, Columbia University, New York, USA (6) Astro-Space Center of Lebedev Physical Institute, 84/32 Profsoyuznaya st., Moscow, 117997, Russia (7) Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, Ontario, M5S 3H8, Canada (8) Sterrewacht Leiden, Leiden University, P.O. Box 9513,NL-2300 RA Leiden, The Netherlands (9) Faculty of Physics, Weizmann Institute of Science, Rehovot 76100, Israel, (10) William Z. and Eda Bess Novick career development chair, (11) School of Physics \& Astronomy, Tel Aviv University, P.O. Box 39040, Tel Aviv 69978, Israel Paper : ApJ, 2006, in press EPrint : astro-ph/0601268 Abstract: We report the definite spectroscopic identification of 40 OB supergiants, giants and main sequence stars in the central parsec of the Galaxy. Detection of their absorption lines have become possible with the high spatial and spectral resolution and sensitivity of the adaptive optics integral field spectrometer SPIFFI/SINFONI on the ESO VLT. Several of these OB stars appear to be helium and nitrogen rich. Almost all of the 80 massive stars now known in the central parsec (central arcsecond excluded) reside in one of two somewhat thick (<|h|/R> 0.14) rotating disks. These stellar disks have fairly sharp inner edges (R 1'') and surface density profiles that scale as R^-2. We do not detect any OB stars outside the central 0.5 pc. The majority of the stars in the clockwise system appear to be on almost circular orbits, whereas most of those in the `counter-clockwise' disk appear to be on eccentric orbits. Based on its stellar surface density distribution and dynamics we propose that IRS 13E is an extremely dense cluster (\rhocore >3*10^8 Msun pc^-3), which has formed in the counter-clockwise disk. The stellar contents of both systems are remarkably similar, indicating a common age of 6 +/-2 Myr. The K-band luminosity function of the massive stars suggests a top-heavy mass function and limits the total stellar mass contained in both disks to 1.5*10^4 Msun. Our data strongly favor in situ star formation from dense gas accretion disks for the two stellar disks. This conclusion is very clear for the clockwise disk and highly plausible for the counter-clockwise system. ------------------------------------------------------------------------ Email : richard.larson@yale.edu Title : Understanding the Stellar Initial Mass Function Author(s): Richard B. Larson Institute: (1) Department of Astronomy, Yale University, New Haven, Connecticut, USA. Paper : Revista Mexicana de Astronomia y Astrofisica, Web : http://www.astro.yale.edu/larson/papers/Pucon05.pdf. Abstract: The essential features of the stellar Initial Mass Function are, rather generally, (1) a peak at mass of a few tenths of a solar mass, and (2) a power-law tail toward higher masses that is similar to the original Salpeter function. Recent work suggests that the IMF peak reflects a preferred scale of fragmentation associated with the transition from a cooling phase of collapse at low densities to a nearly isothermal phase at higher densities, where the gas becomes thermally coupled to the dust. The Salpeter power law is plausibly produced, at least in part, by scale-free accretion processes that build up massive stars in dense environments. The young stars at the Galactic Center appear to have unusually high masses, possibly because of a high minimum mass resulting from the high opacity of the dense star-forming gas. ------------------------------------------------------------------------ Email : ssridhar@rri.res.in Title : Folded Fields as the Source of Extreme Radio-Wave Scattering in the Galactic Center Author(s): Peter Goldreich(1) and S. Sridhar(2) Institute: (1) School of Natural Sciences, Institute for Advanced Study, Einstein Drive, Princeton, NJ 08540 (2) Raman Research Institute, C. V. Raman Avenue, Sadashivanagar, Bangalore 560080, India Paper : APJL, accepted EPrint : astro-ph/0602532 Abstract: A strong case has been made that radio waves from sources within about half a degree of the Galactic Center undergo extreme diffractive scattering. However, problems arise when standard (``Kolmogorov'') models of electron density fluctuations are employed to interpret the observations of scattering in conjunction with those of free-free radio emission. Specifically, the outer scale of a Kolmogorov spectrum of electron density fluctuations is constrained to be so small that it is difficult to identify an appropriate astronomical setting. Moreover, an unacceptably high turbulent heating rate results if the outer scale of the velocity field coincides with that of the density fluctuations. We propose an alternative model based on folded magnetic field structures that have been reported in numerical simulations of small-scale dynamos. Nearly isothermal density variations across thin current sheets suffice to account for the scattering. There is no problem of excess turbulent heating because the outer scale for the velocity fluctuations is much larger than the widths of the current sheets. We speculate that interstellar magnetic fields could possess geometries that reflect their origins: fields maintained by the galactic dynamo could have large correlation lengths, whereas those stirred by local energetic events might exhibit folded structures. ------------------------------------------------------------------------ (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 ========================================================================