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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.
| X-Ray-Emitting Stars Identified from the ROSAT All-Sky Survey and the Sloan Digital Sky Survey
The ROSAT All-Sky Survey (RASS) was the first imaging X-ray survey ofthe entire sky. Combining the RASS Bright and Faint Source Catalogsyields an average of about three X-ray sources per square degree.However, while X-ray source counterparts are known to range from distantquasars to nearby M dwarfs, the RASS data alone are often insufficientto determine the nature of an X-ray source. As a result, large-scalefollow-up programs are required to construct samples of known X-rayemitters. We use optical data produced by the Sloan Digital Sky Survey(SDSS) to identify 709 stellar X-ray emitters cataloged in the RASS andfalling within the SDSS Data Release 1 footprint. Most of these arebright stars with coronal X-ray emission unsuitable for SDSSspectroscopy, which is designed for fainter objects (g > 15 [mag]).Instead, we use SDSS photometry, correlations with the Two Micron AllSky Survey and other catalogs, and spectroscopy from the Apache PointObservatory 3.5 m telescope to identify these stellar X-raycounterparts. Our sample of 707 X-ray-emitting F, G, K, and M stars isone of the largest X-ray-selected samples of such stars. We derivedistances to these stars using photometric parallax relationsappropriate for dwarfs on the main sequence, and use these distances tocalculate LX . We also identify a previously unknowncataclysmic variable (CV) as a RASS counterpart. Separately, we usecorrelations of the RASS and the SDSS spectroscopic catalogs of CVs andwhite dwarfs (WDs) to study the properties of these rarer X-ray-emittingstars. We examine the relationship between (fX /fg) and the equivalent width of the Hβ emission line for 46X-ray-emitting CVs and discuss tentative classifications for a subsetbased on these quantities. We identify 17 new X-ray-emitting DA(hydrogen) WDs, of which three are newly identified WDs. We report onfollow-up observations of three candidate cool X-ray-emitting WDs (oneDA and two DB (helium) WDs); we have not confirmed X-ray emission fromthese WDs.Includes observations obtained with the Apache Point Observatory 3.5 mtelescope, which is owned and operated by the Astrophysical ResearchConsortium.
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