Monday 09/24/2012

The Herschel Virgo Cluster Survey XII: FIR properties of optically-selected Virgo Cluster galaxies

R. Auld, S. Bianchi, M. W. L. Smith, J. I. Davies, G. J. Bendo, S. di Serego Alighieri, L. Cortese, M. Baes, D. J. Bomans, M. Boquien, A. Boselli, L. Ciesla, M. Clemens, E. Corbelli, I. De Looze, J. Fritz, G. Gavazzi, C. Pappalardo, M. Grossi, L. K. Hunt, S. Madden, L. Magrini, M. Pohlen, J. Verstappen, C. Vlahakis, E. M. Xilouris, S. Zibetti
(Submitted on 20 Sep 2012)

The Herschel Virgo Cluster Survey (HeViCS) is the deepest, confusion-limited survey of the Virgo Cluster at far-infrared (FIR) wavelengths. The entire survey at full depth covers $\sim$55 sq. deg. in 5 bands (100-500 \micron), encompassing the areas around the central dominant elliptical galaxies (M87, M86 & M49) and extends as far as the NW cloud, the W cloud and the Southern extension. The survey extends beyond this region with lower sensitivity so that the total area covered is 84 sq. deg. In this paper we describe the data, the data acquisition techniques and present the detection rates of the optically selected Virgo Cluster Catalogue (VCC). We detect 254 (34%) of 750 VCC galaxies found within the survey boundary in at least one band and 171 galaxies are detected in all five bands. For the remainder of the galaxies we have measured strict upper limits for their FIR emission. The population of detected galaxies contains early- as well as late-types although the latter dominate the detection statistics. We have modelled 168 galaxies, showing no evidence of a strong synchrotron component in their FIR spectra, using a single-temperature modified blackbody spectrum with a fixed emissivity index ($\beta = 2$). A study of the $\chi^2$ distribution indicates that this model is not appropriate in all cases, and this is supported by the FIR colours which indicate a spread in $\beta$=1–2. Statistical comparison of the dust mass and temperature distributions from 140 galaxies with $\chi^2_{dof=3} < 7.8$ (95% confidence level) shows that late-types have typically colder, more massive dust reservoirs; the early-type dust masses have a mean of ${\rm log}(<M> / M_{\sun}) = 6.3 \pm 0.3 $, while for late-types ${\rm log}(<M> / M_{\sun}) =7.1 \pm 0.1$… (abridged)

The VLT-FLAMES Tarantula Survey VIII. Multiplicity properties of the O-type star population

H. Sana, A. de Koter, S. E. de Mink, P. R. Dunstall, C. J. Evans, V. Henault-Brunet, J. Maiz Apellaniz, O. H. Ramirez-Agudelo, W. D. Taylor, N. R. Walborn, J. S. Clark, P. A. Crowther, A. Herrero, M. Gieles, N. Langer, D. J. Lennon, J. S. Vink
(Submitted on 20 Sep 2012)

Aims. We analyze the multiplicity properties of the massive O-type star population. With 360 O-type stars, this is the largest homogeneous sample of massive stars analyzed to date.
Methods. We use multi-epoch spectroscopy and variability analysis to identify spectroscopic binaries. We also use a Monte-Carlo method to correct for observational biases.
Results. We observe a spectroscopic binary fraction of 0.35\pm0.03, which corresponds to the fraction of objects displaying statistically significant radial velocity variations with an amplitude of at least 20km/s. We compute the intrinsic binary fraction to be 0.51\pm0.04. We adopt power-laws to describe the intrinsic period and mass-ratio distributions: f_P ~ (log P)^\pi\ (with 0.15 < log P < 3.5) and f_q ~ q^\kappa\ with 0.1 < q=M_2/M_1 < 1.0. The power-law indexes that best reproduce the observed quantities are \pi = -0.45 +/- 0.30 and \kappa = -1.0\pm0.4. The obtained period distribution thus favours shorter period systems compared to an Oepik law. The mass ratio distribution is slightly skewed towards low mass ratio systems but remains incompatible with a random sampling of a classical mass function. The binary fraction seems mostly uniform across the field of view and independent of the spectral types and luminosity classes. The binary fraction in the outer region of the field of view (r > 7.8′, i.e. approx117 pc) and among the O9.7 I/II objects are however significantly lower than expected from statistical fluctuations.
Conclusions. Using simple evolutionary considerations, we estimate that over 50% of the current O star population in 30 Dor will exchange mass with its companion within a binary system. This shows that binary interaction is greatly affecting the evolution and fate of massive stars, and must be taken into account to correctly interpret unresolved populations of massive stars.

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