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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.
| Spitzer 24 ?m Excesses for Bright Galactic Stars in Boötes and First Look Survey Fields
Optically bright Galactic stars (V lsim 13 mag) having f?(24 ?m) > 1 mJy are identified in Spitzermid-infrared surveys within 8.2 deg2 for the Boötesfield of the NOAO Deep Wide-Field Survey and within 5.5 deg2for the First Look Survey (FLS). One hundred and twenty-eight stars areidentified in Boötes and 140 in the FLS, and their photometry isgiven. (K - [24]) colors are determined using K magnitudes from the TwoMicron All Sky Survey for all stars in order to search for excess 24?m luminosity compared to that arising from the stellar photosphere.Of the combined sample of 268 stars, 141 are of spectral types F, G, orK, and 17 of these 141 stars have 24 ?m excesses with (K - [24]) >0.2 mag. Using limits on absolute magnitude derived from proper motions,at least eight of the FGK stars with excesses are main-sequence stars,and estimates derived from the distribution of apparent magnitudesindicate that all 17 are main-sequence stars. These estimates lead tothe conclusion that between 9% and 17% of the main-sequence FGK fieldstars in these samples have 24 ?m infrared excesses. This result isstatistically similar to the fraction of stars with debris disks foundamong previous Spitzer targeted observations of much brighter,main-sequence field stars.
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