The Dynamics of Ultracompact HII Regions Nathaniel Roth Steven W. Stahler Eric Keto Dept. of Physics, U. of California, Berkeley CA 94720 USA Dept. of Astronomy, U. of California, Berkeley, CA 94720 USA Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138 USA Many ultracompact HII regions exhibit a cometary morphology in radio continuum emission. In such regions, a young massive star is probably ablating, through its ultraviolet radiation, the molecular cloud that spawned it. On one side of the star, the radiation drives an ionization front that stalls in dense molecular gas. On the other side, ionized gas streams outward into the more rarefied environment. This wind is underpressured with respect to the neutral gas. The difference in pressure draws in more cloud material, feeding the wind until the densest molecular gas is dissipated. Recent, time-dependent simulations of massive stars turning on within molecular gas show the system evolving in a direction similar to that just described. Here, we explore a semi-analytic model in which the wind is axisymmetric and has already achieved a steady state. Adoption of this simplified picture allows us to study the dependence of both the wind and its bounding ionization front on the stellar luminosity, the peak molecular density, and the displacement of the star from the center of the clump. For typical parameter values, the wind accelerates transonically to a speed of about 15 km/s, and transports mass outward at a rate of 10^{-4} msun/yr. Stellar radiation pressure acts to steepen the density gradient of the wind. Reference: MNRAS Status: Manuscript has been accepted Weblink: http://arxiv.org/abs/1311.5912 Comments: Email: nathaniel.roth@berkeley.edu --- Submitted on Tue Nov 26 16:08:24 CST 2013------------------------- Optical spectra of 5 new Be/X-ray Binaries in the Small Magellanic Cloud and the link of the supergiant B[e] star LHA 115-S 18 with an X-ray source G. Maravelias (1) A. Zezas (1,2,3) V. Antoniou (3,4) D. Hatzidimitriou (5) (1)University of Crete, Physics Department & Institute of Theoretical & Computational Physics, GR-710 03 Heraklion, Crete, Greece (2)Foundation for Research and Technology-Hellas, Institute of Electronic Structure & Laser, GR-711 10 Heraklion, Crete, Greece (3)Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA (4)Iowa State University, Department of Physics & Astronomy, Ames, IA 50011, USA (5)University of Athens, Department of Physics, Section of Astrophysics, Astronomy, and Mechanics, GR-157 84 Zografou, Athens, Greece The Small Magellanic Cloud (SMC) is well known to harbor a large number of High-Mass X-ray Binaries (HMXBs). The identification of their optical counterparts provides information on the nature of the donor stars and can help to constrain the parameters of these systems and their evolution. We obtained optical spectra for a number of HMXBs identified in previous textit{Chandra} and textit{XMM-Newton} surveys of the SMC using the AAOmega/2dF fiber-fed spectrograph at the Anglo-Australian Telescope. We find 5 new Be/X-ray binaries (BeXRBs; including a tentative one), by identifying the spectral type of their optical counterparts, and we confirm the spectral classification of an additional 15 known BeXRBs. We compared the spectral types, orbital periods, and eccentricities of the BeXRB populations in the SMC and the Milky Way and we find marginal evidence for difference between the spectral type distributions, but no statistically significant differences for the orbital periods and the eccentricities. Moreover, our search revealed that the well known supergiant B[e] star LHA 115-S 18 (or AzV 154) is associated with the weak X-ray source CXOU J005409.57-724143.5. We provide evidence that the supergiant star LHA 115-S 18 is the optical counterpart of the X-ray source, and we discuss different possibilities of the origin of its low X-ray luminosity (Lx~4x10^33 erg/s). Reference: MNRAS Status: Manuscript has been accepted Weblink: http://arxiv.org/abs/1312.0593 Comments: Email: gmaravel@physics.uoc.gr --- Submitted on Wed Dec 4 5:47:19 CST 2013------------------------- On the origin of variable structures in the winds of hot luminous stars Yannick J. L. Michaux ^{1, 2}, Anthony F. G. Moffat ^{1}, André-Nicolas Chené ^{3, 4, 5}, Nicole Saint-Louis ^{1}. {1} : Département de physique, Université de Montréal & Centre de Recherche en Astrophysique du Québec {2} École Normale Supérieure de Lyon & Centre de Recherche en Astrophysique de Lyon {3} Departamento de Fisica y Astronomia, Universidad de Valparaiso {4} Departamento de Astronomia, Universidad de Concepcion {5} Gemini Observatory, Northern Operations Center Examination of the temporal variability properties of several strong optical recombination lines in a large sample of Galactic Wolf–Rayet (WR) stars reveals possible trends, especially in the more homogeneous WC than the diverse WN subtypes, of increasing wind variability with cooler subtypes. This could imply that a serious contender for the driver of the variations is stochastic, magnetic subsurface convection associated with the 170 kK partial-ionization zone of iron, which should occupy a deeper and larger zone of greater mass in cooler WR subtypes. This empirical evidence suggests that the heretofore proposed ubiquitous driver of wind variability, radiative instabilities, may not be the only mechanism playing a role in the stochastic multiple small-scaled structures seen in the winds of hot luminous stars. In addition to small-scale stochastic behaviour, subsurface convection guided by a global magnetic field with localized emerging loops may also be at the origin of the large-scale corotating interaction regions as seen frequently in O stars and occasionally in the winds of their descendant WR stars. Reference: On the origin of variable structures in the winds of hot luminous stars Yannick J. L. Michaux; Anthony F. J. Moffat; Andre-Nicolas Chene; Nicole St-Louis Monthly Notices of the Royal Astronomical Society 2013; doi: 10.1093/mnras/stt2102 Status: Manuscript has been accepted Weblink: Comments: Email: michaux@astro.umontreal.ca --- Submitted on Thu Dec 5 10:54:52 CST 2013------------------------- Variability of Massive Stars with Known Spectral Types in the Small Magellanic Cloud Using 8 Years of OGLE-III Data M. Kourniotis, A.Z. Bonanos, I. Soszynski, R. Poleski, G. Krikelis, A. Udalski, M.K. Szymanski, M. Kubiak, G. Pietrzynski, L. Wyrzykowski, K. Ulaczyk, S. Kozlowski, P. Pietrukowicz IAASARS, National Observatory of Athens, Greece & Section of Astrophysics, Astronomy and Mechanics, Faculty of Physics, University of Athens, Greece. We present a variability study of 4646 massive stars in the Small Magellanic Cloud (SMC) with known spectral types from the catalog of Bonanos et al. (2010) using the light curves from the OGLE-III database. The goal is to exploit the time domain information available through OGLE-III to gain insight into the processes that govern the evolution of massive stars. This variability survey of massive stars with known spectral types is larger than any previous survey by a factor of 7. We find that 60% of our sample (2766 stars) show no significant variability and 40% (1880 stars) exhibit variability distributed as follows: 807 stars display low-amplitude stochastic variability with fluctuations in I-band of up to 0.05 mag, 443 stars present irregular variability of higher amplitude (76% of these are reported as variables for the first time), 205 are eclipsing binaries (including 101 newly discovered systems), 50 are candidate rotating variables, 126 are classical Cepheids, 188 stars exhibit short-term sinusoidal periodicity (P < 3 days) making them candidate "slowly pulsating B stars" and non-radial Be pulsators, and 61 periodic stars exhibit longer periods. We demonstrate the wealth of information provided in the time domain, by doubling the number of known massive eclipsing binary systems and identifying 189 new candidate early-type Be and 20 Oe stars in the SMC. In addition, we find that ~80% of Be stars are photometrically variable in the OGLE-III time domain and provide evidence that short-term pulsating stars with additional photometric variability are rotating close to their break-up velocity. Reference: A&A in press. Status: Manuscript has been accepted Weblink: http://arxiv.org/abs/1310.5701 Comments: 46 pages, 18 figures, 11 tables. Email: mkourniotis@astro.noa.gr --- Submitted on Mon Jan 20 23:04:20 CST 2014------------------------- Variability of Massive Stars with Known Spectral Types in the Small Magellanic Cloud Using 8 Years of OGLE-III Data M. Kourniotis (1), A.Z. Bonanos (1), I. Soszynski, R. Poleski, G. Krikelis, A. Udalski, M.K. Szymanski, M. Kubiak, G. Pietrzynski, L. Wyrzykowski, K. Ulaczyk, S. Kozlowski, P. Pietrukowicz (1) National Observatory of Athens, Greece We present a variability study of 4646 massive stars in the Small Magellanic Cloud (SMC) with known spectral types from the catalog of Bonanos et al. (2010) using the light curves from the OGLE-III database. The goal is to exploit the time domain information available through OGLE-III to gain insight into the processes that govern the evolution of massive stars. This variability survey of massive stars with known spectral types is larger than any previous survey by a factor of 7. We find that 60% of our sample (2766 stars) show no significant variability and 40% (1880 stars) exhibit variability distributed as follows: 807 stars display low-amplitude stochastic variability with fluctuations in I-band of up to 0.05 mag, 443 stars present irregular variability of higher amplitude (76% of these are reported as variables for the first time), 205 are eclipsing binaries (including 101 newly discovered systems), 50 are candidate rotating variables, 126 are classical Cepheids, 188 stars exhibit short-term sinusoidal periodicity (P < 3 days) making them candidate "slowly pulsating B stars" and non-radial Be pulsators, and 61 periodic stars exhibit longer periods. We demonstrate the wealth of information provided in the time domain, by doubling the number of known massive eclipsing binary systems and identifying 189 new candidate early-type Be and 20 Oe stars in the SMC. In addition, we find that ~80% of Be stars are photometrically variable in the OGLE-III time domain and provide evidence that short-term pulsating stars with additional photometric variability are rotating close to their break-up velocity. Reference: A&A, in press Status: Manuscript has been accepted Weblink: http://arxiv.org/abs/1310.5701 Comments: 46 pages, 18 figures, 11 tables Email: bonanos@noa.gr --- Submitted on Mon Jan 20 23:25:41 CST 2014-------------------------