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Comeron F.,ESO | Pasquali A.,University of Heidelberg
Astronomy and Astrophysics | Year: 2012

Context. The Cygnus OB2 association and its surroundings display the richest collection of massive stars in our nearby Galactic environment and a wealth of signposts of the interaction between these stars and the interstellar gas. Aims. We perform a magnitude-limited, homogeneous census of O and early B-type stars with accurate spectral classifications in the blue, in a 6° × 4° region centered on Cygnus OB2 that includes most of the Cygnus X complex, a sizeable fraction of the adjacent Cygnus OB9 association, and a large area of the field surrounding these complexes. Methods. By using reddening-free indices based on BJHK magnitudes from the USNO-B and 2MASS catalogs, we are able to produce a highly complete, highly uncontaminated sample of O and early B stars, which nearly duplicates any previous census of the region for the same range of spectral types. We provide the spectral types of 60 new O and B stars, as well as a list of an additional 60 candidates pending spectroscopic confirmation. In addition, the UBV imaging of the surroundings of three apparently isolated O stars is used to investigate the possible presence of small clusters of young stars around them. Results. Early-type stars are consistent with similar distances for Cygnus OB2, OB9, and the field stars surrounding them. We confirm previous findings of an older population in Cygnus OB2 spatially offset from where the stellar density of the association peaks. Some new remarkable objects are identified, including BD+40 4210, a B0 supergiant member of Cygnus OB2 that is among the brightest members of the association sharing some characteristics with luminous blue variable (LBV) candidates, located at a projected distance of 5 pc from another LBV candidate. A new O5If member of Cygnus OB9 is found, as well as several other O stars and B supergiants. On the other hand, while no obvious clustering is found around the apparently isolated O stars, the fields around two of them seem to contain objects with strong ultraviolet excesses, which perhaps indicates that they are accreting, although their nature and possible relationship to the O stars in the field are unclear. Conclusions. Star formation in Cygnus has been taking place in a sustained manner for well over 10 Myr, with a large-scale trend of proceeding from lower to higher Galactic longitudes. Star formation inside Cygnus OB2 follows this trend, with indications of intense star formation activity having started in the southern (lower galactic latitude) part of the association about 10 Myr ago and probably continuing at present in the north. © 2012 ESO. Source

Moeckel N.,University of Cambridge | Goddi C.,ESO
Monthly Notices of the Royal Astronomical Society | Year: 2012

The Orion BN/KL complex is the nearest site of ongoing high-mass star formation. Recent proper motion observations provide convincing evidence of a recent (about 500 years ago) dynamical interaction between two massive young stellar objects (YSOs) in the region resulting in high velocities: the BN object and radio Source I. At the same time, Source I is surrounded by a nearly edge-on disc with radius ∼50 au. These two observations taken together are puzzling: a dynamical encounter between multiple stars naturally yields the proper motions, but the survival of a disc is challenging to explain. In this paper we take the first steps to numerically explore the preferred dynamical scenario of Goddi et al., in which Source I is a binary that underwent a scattering encounter with BN, in order to determine if a pre-existing disc can survive this encounter in some form. Treating only gravitational forces, we are able to thoroughly and efficiently cover a large range of encounter parameters. We find that disc material can indeed survive a three-body scattering event if (1) the encounter is close, i.e. BN's closest approach to Source I is comparable to Source I's semimajor axis and (2) the interplay of the three stars is of a short duration. Furthermore, we are able to constrain the initial conditions that can broadly produce the orientation of the present-day system's disc relative to its velocity vector. To first order we can thus confirm the plausibility of the scattering scenario of Goddi et al., and we have significantly constrained the parameters and narrowed the focus of future, more complex and expensive attempts to computationally model the complicated BN/KL region. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS. Source

Boffin H.M.J.,ESO
Astronomy and Astrophysics | Year: 2010

Aims. Triggered by the study of Carquillat & Prieur (2007, MNRAS, 380, 1064) of Am binaries, I reanalyse their sample of 60 orbits to derive the mass ratio distribution (MRD), assuming as they did a priori functional forms, i.e. a power law or a Gaussian. The sample is then extended using orbits published by several groups and a full analysis of the MRD is made, without any assumption on the functional form. Methods. I derive the MRD using a Richardson-Lucy inversion method, assuming a fixed mass of the Am primary and randomly distributed orbital inclinations. Using the large sub-sample of double-lined spectroscopic binaries, I show that this methodology is indeed perfectly adequate. Results. I first derive new parameters of the functional form for the Carquillat & Prieur sample. Using the inversion method, applied to my extended sample of 162 systems, I find that the final MRD can be approximated by a uniform distribution. © 2010 ESO. Source

Rejkuba M.,ESO
Astrophysics and Space Science | Year: 2012

Globular clusters are among the first objects used to establish the distance scale of the Universe. In the 1970-ies it has been recognized that the differential magnitude distribution of old globular clusters is very similar in different galaxies presenting a peak at MV~-7. 5. This peak magnitude of the so-called Globular Cluster Luminosity Function has been then established as a secondary distance indicator. The intrinsic accuracy of the method has been estimated to be of the order of ~0. 2 mag, competitive with other distance determination methods. Lately the study of the Globular Cluster Systems has been used more as a tool for galaxy formation and evolution, and less so for distance determinations. Nevertheless, the collection of homogeneous and large datasets with the ACS on board HST presented new insights on the usefulness of the Globular Cluster Luminosity Function as distance indicator. I discuss here recent results based on observational and theoretical studies, which show that this distance indicator depends on complex physics of the cluster formation and dynamical evolution, and thus can have dependencies on Hubble type, environment and dynamical history of the host galaxy. While the corrections are often relatively small, they can amount to important systematic differences that make the Globular Cluster Luminosity Function a less accurate distance indicator with respect to some other standard candles. © 2012 Springer Science+Business Media B.V. Source

Hatziminaoglou E.,ESO | Fritz J.,Ghent University
Monthly Notices of the Royal Astronomical Society | Year: 2012

The geometry of dust distribution within the inner regions of active galactic nuclei (AGN) is still a debated issue and relates directly to the AGN unified scheme. Traditionally, models discussed in the literature assume one of the two distinct dust distributions in what is believed to be a toroidal region around the supermassive black holes: a continuous distribution, customarily referred to as smooth, and a concentration of dust in clumps or clouds, referred to as clumpy. In this paper we perform a thorough comparison between two of the most popular models in the literature, namely the smooth models by Fritz et al. and the clumpy models by Nenkova et al., in their common parameter space. Particular attention is paid to the silicate features at ∼9.7 and ∼18μm, the width of the infrared bump, the near-infrared index and the luminosity at 12.3μm, all previously reported as possible diagnostic tools to distinguish between the two dust distributions. We find that, due to different dust chemical compositions used in the two models, the behaviour of the silicate features at 9.7 and 18μm is quite distinct between the two models. The width of the infrared bump and the peak of the infrared emission can take comparable values; their distributions do, however, vary. The near-infrared index is also quite different, due partly to the primary sources adopted by the two models. Models with matched parameters do not produce similar spectral energy distributions (SEDs) and virtually no random parameter combinations can result in seemingly identical SEDs. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS. Source

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