======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 29, No. 8 May 22, 2009 Recently submitted papers: -------------------------- 1) Characteristics of the Galaxy according to Cepheids (Majaess et al., MNRAS) 2) HII regions, infrared dark molecular clouds and the local geometry of the Milky Way's nuclear star-forming ring (Liszt, A&A) 3) Detecting Flaring Structures in Sagittarius A* with High-Frequency VLBI (Doeleman et al., ApJ) 4) Star Formation in the Central 400 pc of the Milky Way: Evidence for a Population of Massive YSOs (Yusef-Zadeh et al., ApJ) ------------------------------------------------------------------------ Email : dmajaess@ap.stmarys.ca Title : Characteristics of the Galaxy according to Cepheids Author(s): D. J. Majaess^1,2, D. G. Turner^1 and D. J. Lane^1,2 Institute: (^1) Department of Astronomy and Physics, Saint Mary's University, Halifax, Nova Scotia B3H 3C3, Canada (^2) The Abbey Ridge Observatory, Stillwater Lake, Nova Scotia, Canada Paper : MNRAS, May 2009, in press EPrint : 0903.4206 Abstract: Classical and Type II Cepheids are used to reinvestigate specific properties of the Galaxy. A new Type II reddening-free Cepheid distance parameterization is formulated from LMC Cepheids (OGLE), with uncertainties typically no larger than 5-15%. A distance to the Galactic centre of R_0 = 7.8+/-0.6 kpc is derived from the median distance to Type II Cepheids in the bulge (OGLE), R_0 = 7.7+/-0.7 kpc from a distance to the near side of the bulge combined with an estimated bulge radius of 1.3+/-0.3 kpc derived from planetary nebulae. The distance of the Sun from the Galactic plane inferred from classical Cepheid variables is Z_\sun=26+/-3 pc, a result dependent on the sample's distance and direction because of the complicating effects of Gould's Belt and warping in the Galactic disk. Classical Cepheids and young open clusters delineate consistent and obvious spiral features, although their characteristics do not match conventional pictures of the Galaxy's spiral pattern. The Sagittarius-Carina arm is confirmed as a major spiral arm that appears to originate from a different Galactic region than suggested previously. Furthermore, a major feature is observed to emanate from Cygnus-Vulpecula and may continue locally near the Sun. Significant concerns related to the effects of metallicity on the VI-based reddening-free Cepheid distance relations used here are allayed by demonstrating that the computed distances to the Galactic centre, and to several globular clusters (M54, NGC 6441, M15, and M5) and galaxies (NGC 5128 and NGC 3198) which likely host Type II Cepheids: agree with literature results to within the uncertainties. An additional empirical test is proposed to constrain any putative metallicity dependence of Cepheid distance determinations through forced matches of distance estimates to a particular galaxy using both Type II and classical Cepheids. ------------------------------------------------------------------------ Email : hliszt@nrao.edu Title : HII regions, infrared dark molecular clouds and the local geometry of the Milky Way's nuclear star-forming ring Author(s): H. S. Liszt(1) Institute: (1) National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA, USA 22903-2475 Paper : A&A, submitted 9 May 2009 EPrint : 0905.1412 Abstract: Observations of the HII region-molecular cloud complexes Sgr B, C, D and E provide unique insights into the structure of a nuclear star-forming ring. To interpret the galactic center HII region/molecular cloud complexes as parts of a barred galaxy's nuclear star-forming ring. We compare observations of 18cm VLA radiocontinuumm, 8-22 mu MSX IR and 2.6mm BTL/KP12m CO emission from the Sgr B, C, D and E complexes in the inner few degrees of longitude. The observed IR structure and 8-22 mu IR source spectra are heavily influenced by extinction in individual compact HII regions and over entire source complexes: in the most striking case, the powerful compact HII regions in Sgr B2 at l > 0.6 DEGr are almost undetected by MSX. In a few especially favorable cases the IR extinction from individual galactic center molecular clouds is seen to be wavelength-independent. The kinematics of the HII regions are mostly rotational, as expected for a nuclear star-forming ring but with spectactular departures in Sgr B and Sgr C. Owing to high molecular column density and flattening of the extinction curve, the galactic center HII regions would likely be shrouded even if viewed from outside. Slight departures from circular motion and perfect alignment with the galactic equator allow the front (Sgr B) and rear (Sgr C) portions of the Galaxy's nuclear star-forming ring to be distinguished. Much of the volume interior to the ring may be somewhat hollow. ------------------------------------------------------------------------ Email : vfish@franklin.haystack.mit.edu Title : Detecting Flaring Structures in Sagittarius A* with High-Frequency VLBI Author(s): Sheperd S. Doeleman(1), Vincent L. Fish(1), Avery E. Broderick(2), Abraham Loeb(3), Alan E. E. Rogers(1) Institute: (1) Massachusetts Institute of Technology, Haystack Observatory, Route 40, Westford, MA 01886. (2) Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George St., Toronto, ON, M5S 3H8 Canada. (3) Institute for Theory and Computation, Harvard University, Center for Astrophysics, 60 Garden St., Cambridge, MA 02138. Paper : ApJ, 2009, 695, 59 EPrint : 0809.3424 Abstract: The super-massive black hole candidate, Sagittarius A*, exhibits variability from radio to X-ray wavelengths on timescales that correspond to < 10 Schwarzschild radii. We survey the potential of millimeter wavelength very long baseline interferometry (VLBI) to detect and constrain time-variable structures that could give rise to such variations, focusing on a model in which an orbiting hot spot is embedded in an accretion disk. Nonimaging algorithms are developed that use interferometric closure quantities to test for periodicity, and applied to an ensemble of hot spot models that sample a range of parameter space. We find that structural periodicity in a wide range of cases can be detected on most potential VLBI arrays using modern VLBI instrumentation. Future enhancements of millimeter/submillimeter VLBI arrays including phased-array processors to aggregate VLBI station collecting area, increased bandwidth recording, and addition of new VLBI sites all significantly aid periodicity detection. The methods described herein can be applied to other models of Sagittarius A*, including jet outflows and magnetohydrodynamic accretion simulations. ------------------------------------------------------------------------ Email : zadeh@northwestern.edu Title : Star Formation in the Central 400 pc of the Milky Way: Evidence for a Population of Massive YSOs Author(s): Yusef-Zadeh, F.; Hewitt, J.; Arendt, R. G.; Whitney, B.; Rieke, G.; Wardle, M.; Hinz, J. L.; Stolovy, S.; Lang, C. C.; Burton, M. G. and Ramirez, S. Paper : ApJ, May 2009 Abstract: The central kpc of the Milky Way might be expected to differ significantly from the rest of the Galaxy with regard to gas dynamics and the formation of young stellar objects (YSOs). We probe this possibility with mid-infrared observations obtained with IRAC and MIPS on Spitzer and with MSX. We use color-color diagrams and spectral energy distribution (SED) fits to explore the nature of YSO candidates (including objects with 4.5micron excesses possibly due to molecular emission). There is an asymmetry in the distribution of the candidate YSOs, which tend to be found at negative Galactic longitudes; this behavior contrasts with that of the molecular gas, approximately 2/3 of which is at positive longitudes. The small scale height of these objects suggests that they are within the Galactic center region and are dynamically young. They lie between two layers of infrared dark clouds (IRDCs) and may have originated from these clouds. We identify new sites for this recent star formation by comparing the mid-IR, radio, submillimeter, and methanol maser data. The methanol masers appear to be associated with young, embedded YSOs characterized by 4.5micron excesses. We use the SEDs of these sources to estimate their physical characteristics; their masses appear to range from 10 to 20 \msol. Within the central 400x50 pc (|l|<1.3 DEGr and | b|<10') the star formation rate based on the identification of Stage I evolutionary phase of YSO candidates is about 0.14 solar mass/yr. Given that the majority of the sources in the population of YSOs is classified as Stage I objects, we suggest that a recent burst of star formation took place within the last 10^5 years. This suggestion is also consistent with estimates of star formation rates within the last 10^7 years showing a peak around 10^5 years ago. Lastly, we find that the Schmidt-Kennicutt Law applies well in the central 400 pc of the Galaxy. This implies that star formation does not appear to be dramatically affected by the extreme physical conditions in the Galactic center region. ------------------------------------------------------------------------ (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 ========================================================================