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Bayesian inference of stellar parameters and interstellar extinction using parallaxes and multiband photometry
Astrometric surveys provide the opportunity to measure the absolutemagnitudes of large numbers of stars, but only if the individualline-of-sight extinctions are known. Unfortunately, extinction is highlydegenerate with stellar effective temperature when estimated frombroad-band optical/infrared photometry. To address this problem, Iintroduce a Bayesian method for estimating the intrinsic parameters of astar and its line-of-sight extinction. It uses both photometry andparallaxes in a self-consistent manner in order to provide anon-parametric posterior probability distribution over the parameters.The method makes explicit use of domain knowledge by employing theHertzsprung-Russell Diagram (HRD) to constrain solutions and to ensurethat they respect stellar physics. I first demonstrate this method byusing it to estimate effective temperature and extinction from BVJHKdata for a set of artificially reddened Hipparcos stars, for whichaccurate effective temperatures have been estimated from high-resolutionspectroscopy. Using just the four colours, we see the expected strongdegeneracy (positive correlation) between the temperature andextinction. Introducing the parallax, apparent magnitude and the HRDreduces this degeneracy and improves both the precision (reduces theerror bars) and the accuracy of the parameter estimates, the latter byabout 35 per cent. The resulting accuracy is about 200 K in temperatureand 0.2 mag in extinction. I then apply the method to estimate theseparameters and absolute magnitudes for some 47 000 F, G, K Hipparcosstars which have been cross-matched with Two-Micron All-Sky Survey(2MASS). The method can easily be extended to incorporate the estimationof other parameters, in particular metallicity and surface gravity,making it particularly suitable for the analysis of the 109stars from Gaia.
| The outskirts of Cygnus OB2
Context: Cygnus OB2 is one of the richest OB associations in the localGalaxy, and is located in a vast complex containing several otherassociations, clusters, molecular clouds, and HII regions. However, thestellar content of Cygnus OB2 and its surroundings remains rather poorlyknown largely due to the considerable reddening in its direction atvisible wavelength. Aims: We investigate the possible existence of anextended halo of early-type stars around Cygnus OB2, which is hinted atby near-infrared color-color diagrams, and its relationship to CygnusOB2 itself, as well as to the nearby association Cygnus OB9 and to thestar forming regions in the Cygnus X North complex. Methods: Candidateselection is made with photometry in the 2MASS all-sky point sourcecatalog. The early-type nature of the selected candidates is confirmedor discarded through our infrared spectroscopy at low resolution. Inaddition, spectral classifications in the visible are presented for manylightly-reddened stars. Results: A total of 96 early-type stars areidentified in the targeted region, which amounts to nearly half of theobserved sample. Most of them have featureless near-infrared spectra asexpected from OB stars at the available resolution. Another 18 starsthat display Brackett emission lines can be divided between evolvedmassive stars (most likely Be stars) and Herbig Ae/Be stars based ontheir infrared excesses. A component associated with Cygnus OB9/NGC 6910is clearly identified, as well as an enhancement in the surface densityof early-type stars at Cygnus X North. We also find a field population,consisting largely of early B giants and supergiants, which is probablythe same as identified in recent studies of the inner 1° circlearound Cygnus OB2. The age and large extension of this populationdiscards a direct relationship with Cygnus OB2 or any other particularassociation. Conclusions: Earlier claims of the possible large extentof Cygnus OB2 beyond its central, very massive aggregate seem to bedismissed by our findings. The existence of a nearly ubiquitouspopulation of evolved stars with massive precursors suggests a massivestar formation history in Cygnus having started long before theformation of the currently observed OB associations in the region.Based on observations collected at the Centro AstronómicoHispano-Alemán (CAHA) at Calar Alto, operated jointly by theMax-Planck Institut für Astronomie and the Instituto deAstrofísica de Andalucía (CSIC).Tables 1-9 are only available in electronic form at http://www.aanda.org
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