======================================================================== 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. 1 Feb 19, 2008 Recently submitted papers: -------------------------- 1) GBT Multiwavelength Survey of the Galactic Center Region (Law et al., ApJS) 2) Cataclysmic Variables in Globular Clusters, the Galactic Center, and Local Space (Heinke et al., Proceedings) 3) The distance to the Galactic Centre based on Population ii Cepheids and RR Lyrae stars (Groenewegen et al., A&A) 4) Hypervelocity Stars: From the Galactic Center to the Halo (Kenyon et al., ApJ) 5) Spectral imaging of the Sagittarius B2 region in multiple 3-mm molecular lines with the Mopra telescope (Jones et al., MNRAS) 6) LMC origin of the hyper-velocity star HE 0437-5439. Beyond the supermassive black hole paradigm (Przybilla et al., A&A) 7) Tracing shocks and photodissociation in the Galactic center region (Martin et al., ApJ) 8) Tracing intermediate-mass black holes in the Galactic Centre (Baumgardt, MNRAS) ------------------------------------------------------------------------ Email : caseyjlaw@gmail.com Title : GBT Multiwavelength Survey of the Galactic Center Region Author(s): C. J. Law(1,2), F. Yusef-Zadeh(1), W. D. Cotton(3), and R. J. Maddalena(4) Institute: (1) Department of Physics and Astronomy, Northwestern University, 2145 Sheriadan Road, Evanston, 60208 IL, USA (2) Astronomical Institute ``Anton Pannekoek'', University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, Netherlands (3) National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA (4) National Radio Astronomy Observatory, P.O. Box 2, Rt. 28/92, Green Bank, WV 24944, USA Paper : ApJS, July 2008 , in press EPrint : 0801.4294 Abstract: We describe the results of a radio continuum survey of the central 4\sdeg*1\sdeg with the 100 m Green Bank Telescope (GBT) at wavelengths of 3.5, 6, 20, and 90 cm.  The 3.5 and 6 cm surveys are the most sensitive and highest resolution single dish surveys made of the central degrees of our Galaxy.  We present catalogs of compact and extended sources in the central four degrees of our Galaxy, including detailed spectral index studies of all sources.  The analysis covers star-forming regions such as Sgr B and Sgr C where we find evidence of a mixture of thermal and nonthermal emission.  The analysis quantifies the relative contribution of thermal and nonthermal processes to the radio continuum flux density toward the GC region.  In the central 4\sdeg*1\sdeg of the GC, the thermal and nonthermal flux fractions for all compact and diffuse sources are 28%/72% at 3.5 cm and 19%/81% at 6 cm.  The total flux densities from these sources are 783+/-52 Jy and 1063+/-93 Jy at 3.5 and 6 cm, respectively, excluding the contribution of Galactic synchrotron emission. ------------------------------------------------------------------------ Email : coh5z@virginia.edu Title : Cataclysmic Variables in Globular Clusters, the Galactic Center, and Local Space Author(s): Craig O. Heinke Ashley J. Ruiter Michael P. Muno Krzysztof Belczynski Institute: (1) University of Virginia, Astronomy Dept., PO Box 400325, Charlottesville VA 22903 (2) Dept. of Astronomy, New Mexico State University, 1320 Frenger Mall, Las Cruces, NM 88003 (3) Space Radiation Laboratory, California Institute of Technology, Pasadena, CA 91125 (4) Dept. of Astronomy, New Mexico State University, 1320 Frenger Mall, Las Cruces, NM 88003; Tombaugh Fellow Paper : Proceedings of "A Population Explosion", R. EPrint : 0801.1279 Abstract: We compare the X-ray spectra and luminosities, in the 2-8 keV band, of known and suspected cataclysmic variables (CVs) in different environments, assessing the nature of these source populations. These objects include nearby CVs observed with ASCA; the Galactic Center X-ray source population identified by Muno et al.; and likely CVs identified in globular clusters. Both of the latter have been suggested to be dominated by magnetic CVs. We find that the brighter objects in both categories are likely to be magnetic CVs, but that the fainter objects are likely to include a substantial contribution from normal CVs. The strangely hard spectra observed from the Galactic Center sources reflect the high and variable extinction, which is significantly greater than the canonical 6*10^22 cm^-2 over much of the region, and the magnetic nature of many of the brightest CVs. The total numbers of faint Galactic Center sources are compatible with expectations of the numbers of CVs in this field. ------------------------------------------------------------------------ Email : groen@ster.kuleuven.be Title : The distance to the Galactic Centre based on Population ii Cepheids and RR Lyrae stars Author(s): M.A.T. Groenewegen (1) A. Udalski (2) and G. Bono (3,4) Institute: (1) Instituut voor Sterrenkunde, Celestijnenlaan 200 D, B-3001 Leuven, Belgium, and Warsaw University Observatory, Aleje Ujazdowskie 4, PL-00-478, Warsaw, Poland and INAF Osservatorio Astronomico di Roma, Via Frascati 33, I-00040 Monte Porzio Catone, Italy and European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching bei Munchen, Germany Paper : A&A accepted EPrint : 0801.2652 Abstract: The distance to the Galactic Centre (GC) is of importance for the distance scale in the Universe. The value derived by Eisenhauer et al. (2005) of 7.62 +/- 0.32 kpc based on the orbit of one star around the central black hole is shorter than most other distance estimates based on a variety of different methods. To establish an independent distance to the GC with high accuracy. To this end Population- ii Cepheids are used that have been discovered in the \OG- ii and \OG- iii surveys. Thirty-nine Population- ii Cepheids have been monitored with the SOFI infrared camera on 4 nights spanning 14 days, obtaining typically between 5 and 11 epochs of data. Light curves have been fitted using the known periods from the OGLE data to determine the mean K-band magnitude with an accuracy of 0.01-0.02 mag. It so happens that 37 RR Lyrae stars are in the field-of-view of the observations and mean K-band magnitudes are derived for this sample as well. After correction for reddening, the period-luminosity relation of Population- ii Cepheids in the K-band is determined, and the derived slope of -2.24 +/- 0.14 is consistent with the value derived by Matsunaga et al. (2006). Fixing the slope to their more accurate value results in a zero point, and implies a distance modulus to the GC of 14.51 +/- 0.12, with an additional systematic uncertainty of 0.07 mag. Similarly, from the RR Lyrae K-band period-luminosity relation we derive a value of 14.48 +/- 0.17 (random) +/- 0.07 (syst.). The two independent determinations are averaged to find 14.50 +/- 0.10 (random) +/- 0.07 (syst.), or 7.94 +/- 0.37 +/- 0.26 kpc. The absolute magnitude scale of the adopted period-luminosity relations is tied to an LMC distance modulus of 18.50 +/- 0.07. ------------------------------------------------------------------------ Email : kenyon@cfa.harvard.edu Title : Hypervelocity Stars: From the Galactic Center to the Halo Author(s): Scott J. Kenyon(1), Benjamin C. Bromley(2), Margaret J. Geller(1), Warren R. Brown(1) Institute: (1) Smithsonian Astrophysical Observatory, , 60 Garden St., Cambridge, MA 02138 (2) Department of Physics, University of Utah, , 115 S 1400 E, Rm 201, Salt Lake City, UT 84112 Paper : ApJ, submitted EPrint : 0801.3479 Abstract: Hypervelocity stars (HVS) traverse the Galaxy from the central black hole to the outer halo. We show that the Galactic potential within 200 pc acts as a high pass filter preventing low velocity HVS from reaching the halo. To trace the orbits of HVS throughout the Galaxy, we construct two forms of the potential which reasonably represent the observations in the range 5-10^5 pc, a simple spherically symmetric model and a bulge-disk-halo model. We use the Hills mechanism (disruption of binaries by the tidal field of the central black hole) to inject HVS into the Galaxy and compute the observable spatial and velocity distributions of HVS with masses in the range 0.6-4 M_o. These distributions reflect the mass function in the GC, properties of binaries in the GC, and aspects of stellar evolution and the injection mechanism. For 0.6-4 \msun main sequence stars, the fraction of unbound HVS and the asymmetry of the velocity distribution for their bound counterparts increases with stellar mass. The density profiles for unbound HVS decline with distance from the GC approximately as r^-2 (but are steeper for the most massive stars which evolve off the main sequence during their travel time from the GC); the density profiles for the bound ejecta decline with distance approximately as r^-3. In a survey with limiting magnitude V <= 23, the detectability of HVS (unbound or bound) increases with stellar mass. ------------------------------------------------------------------------ Email : pjones@phys.unsw.edu.au Title : Spectral imaging of the Sagittarius B2 region in multiple 3-mm molecular lines with the Mopra telescope Author(s): P. A. Jones^1, M. G. Burton^1, M. R. Cunningham^1, K. M. Menten^2, P. Schilke^2, A. Belloche^2, S. Leurini^3, J. Ott^4, A. J. Walsh^5 Institute: ^1 School of Physics, University of New South Wales, NSW 2052, Australia, ^2 Max-Planck-Institut fuer Radioastronomie, Auf dem Huegel 69, 53121 Bonn, Germany, ^3 European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany, ^4 National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA, ^5 School of Maths, Physics and IT, James Cook University, Qld 4814, Australia, Paper : MNRAS, in press EPrint : 0712.0218 Abstract: Using the Mopra telescope, we have undertaken a 3-mm spectral-line imaging survey of a 5 arcmin square area around Sgr B2. We covered almost the complete spectral the range from 81.7 to 113.5 GHz, with 2.2 MHz wide spectral channels or 6 km s^-1 and have observed 24 lines, with 0.033 MHz wide, or 0.1 km s^-1 channels. We discuss the distribution of around 50 lines, and present velocity-integrated emission images for 38 of the lines. In addition, we have detected around 120 more lines, mostly concentrated at the particularly spectral line-rich Sgr B2(N) source. There are significant differences in molecular emission, pointing to both abundance and excitation differences throughout the region. Seven distinct spatial locations are identified for the emitting species, including peaks near the prominent star forming cores of Sgr B2(N), (M) and (S) that are seen in IR-to-radio continuum images. The other features are a 'North Ridge' and a 'North Cloud' to the north of the Sgr B2 N-M-S cores, a 'South-East Peak' and a 'West Ridge'. The column density, as evident through C^18O, peaks at the Sgr B2(N) and (M) cores, where strong absorption is also evident in otherwise generally-bright lines such as HCO^+, HCN and HNC. Most molecules trace a ridge line to the west of the Sgr B2 N-M-S cores, wrapping around the cores and extending NE to the North Cloud. This is most clearly evident in the species HC_3N, CH_3CN, CH_3OH and OCS. They are found to be closer in distribution to the cooler dust traced by the sub-mm continuum than either the warmer dust seen in the mid-IR or to the radio continuum. The molecule CN, in contrast, is reasonably uniform over the entire region mapped, aside from strong absorption at the positions of the Sgr B2(N) and (M) cores. ------------------------------------------------------------------------ Email : przybilla@sternwarte.uni-erlangen.de Title : LMC origin of the hyper-velocity star HE 0437-5439. Beyond the supermassive black hole paradigm Author(s): N. Przybilla(1), M. F. Nieva(1), U. Heber(1), M. Firnstein(1), K. Butler(2), R. Napiwotzki(3), H. Edelmann(1) Institute: (1) Dr. Remeis-Sternwarte Bamberg, Sternwartstr. 7, D-96049 Bamberg, Germany (2) Universitaetssternwarte Muenchen, Scheinerstr. 1, D-81679 Muenchen, Germany (3) Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK Paper : A&A, accepted Web : http://www.sternwarte.uni-erlangen.de/~ai32/hvs.pdf Abstract: Hyper-velocity stars move so fast that only a supermassive black hole (SMBH) seems to be capable to accelerate them. Hence the Galactic centre (GC) is their only suggested place of origin. Edelmann et al. (2005) found the early B-type star HE 0437-5439 to be too short-lived to have reached its current position in the Galactic halo if ejected from the GC, except if being a blue straggler star. Its proximity to the Large Magellanic Cloud (LMC) suggested an origin from this galaxy. The chemical signatures of stars at the GC are significantly different from those in the LMC. Hence, an accurate measurement of the abundance pattern of HE 0437-5439 will yield a new tight constraint on the place of birth of this hyper-velocity star. High-resolution spectra obtained with UVES on the VLT are analysed using state-of-the-art non-LTE modelling techniques. We measured abundances of individual elements to very high accuracy in HE 0437-5439 as well as in two reference stars, from the LMC and the solar neighbourhood, respectively. The abundance pattern is not consistent at all with that observed in stars near the GC, ruling our an origin from the GC. However, there is a high degree of consistency with the LMC abundance pattern. Our abundance results cannot rule out an origin in the outskirts of the Galactic disk. However, we find the life time of HE 0437-5439 to be more than three times shorter than the time of flight to the edge of the disk, rendering a Galactic origin unlikely. Only one SMBH is known to be present in Galaxy and none in the LMC. Hence the exclusion of an GC origin challenges the SMBH paradigm. We conclude that there must be other mechanism(s) to accelerate stars to hyper-velocity speed than the SMBH. We draw attention to dynamical ejection from dense massive clusters, that has recently been proposed by Gvaramadze et al. (2008). ------------------------------------------------------------------------ Email : smartin@cfa.harvard.edu Title : Tracing shocks and photodissociation in the Galactic center region Author(s): Sergio Martin(1), M.A. Requena-Torres(2), J. Martin-Pintado(3), R. Mauersberger(4) Institute: (1) Harvard-Smithsonian Center for Astrophysics, 60 Garden St., 02138, Cambridge, MA, USA (2) Max-Plack-Institut fuer Radioastronomie, Auf dem Huegel 69, D-53121 Bonn, Germany (3) Departamento de Astrofisica Molecular e Infrarroja, Instituto de Estructura de la Materia, CSIC, Serrano 121, E-28006 Madrid, Spain (4) Instituto de Radioastronomia Milimetrica, Avenida Divina Pastora 7, Local 20, E-18012 Granada, Spain Paper : ApJ accepted Abstract: We present a systematic study of the HNCO, C^18O, ^13CS, and C^34S emission towards 13 selected molecular clouds in the Galactic center region The molecular emission in these positions are used as templates of the different physical and chemical processes claimed to be dominant in the circumnuclear molecular gas of galaxies. The relative abundance of HNCO shows a variation of more than a factor of 20 among the observed sources. The HNCO/^13CS abundance ratio is highly contrasted (up to a factor of 30) between the shielded molecular clouds mostly affected by shocks, where HNCO is released to gas-phase from grain mantles, and those pervaded by an intense UV radiation field, where HNCO is photo-dissociated and CS production favored via ion reactions. We propose the relative HNCO to CS abundance ratio as a highly contrasted diagnostic tool to distinguish between the influence of shocks and/or the radiation field in the nuclear regions of galaxies and their relation to the evolutionary state of their nuclear star formation bursts. ------------------------------------------------------------------------ Email : uloeck@astro.uni-bonn.de Title : Tracing intermediate-mass black holes in the Galactic Centre Author(s): U. Loeckmann and H. Baumgardt Institute: Argelander Institute for Astronomy, University of Bonn, Auf dem Huegel 71, 53121 Bonn, Germany Paper : MNRAS, 384, 323-330, 2008 EPrint : 0711.1326 Web : http://www.blackwell-synergy.com/doi/abs/10.1111/j.1365-2966. 2007.12699.x Abstract: We have developed a new method for post-Newtonian, high-precision integration of stellar systems containing a super-massive black hole (SMBH), splitting the forces on a particle between a dominant central force and perturbations. We used this method to perform fully collisional N-body simulations of inspiralling intermediate-mass black holes (IMBHs) in the centre of the Milky Way. We considered stellar cusps of different power-law indices and analysed the effects of IMBHs of different masses, all starting from circular orbits at an initial distance of 0.1 pc. Our simulations show how IMBHs deplete the central cusp of stars, leaving behind a flatter cusp with slope consistent with what has recently been observed. If an additional IMBH spirals into such a flat cusp, it can take 50 Myr or longer to merge with the central SMBH, thus allowing for direct observation in the near future. The final merger of the two black holes involves gravitational wave radiation which may be observable with planned gravitational wave detectors. Furthermore, our simulations reveal detailed properties of the hyper-velocity stars (HVSs) created, and how generations of HVSs can be used to trace IMBHs in the Galactic centre. We find that significant rotation of HVSs (which would be evidence for an IMBH) can only be expected among very fast stars (v > 1000 km/s). Also, the probability of creating a hyper-velocity binary star is found to be very small. ------------------------------------------------------------------------ (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 ========================================================================