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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.
| uvby-? photometry of solar twins . The solar colors, model atmospheres, and the Teff and metallicity scales
Aims: Solar colors have been determined on the uvby-?photometric system to test absolute solar fluxes, to examine colorspredicted by model atmospheres as a function of stellar parameters(Teff, log g, [Fe/H]), and to probe zero-points ofTeff and metallicity scales. Methods: New uvby-?photometry is presented for 73 solar-twin candidates. Most stars of oursample have also been observed spectroscopically to obtain accuratestellar parameters. Using the stars that most closely resemble the Sun,and complementing our data with photometry available in the literature,the solar colors on the uvby-? system have been inferred. Our solarcolors are compared with synthetic solar colors computed from absolutesolar spectra and from the latest Kurucz (ATLAS9) and MARCS modelatmospheres. The zero-points of different Teff andmetallicity scales are verified and corrections are proposed. Results: Our solar colors are (b-y)? = 0.4105 ±0.0015, m1, ? = 0.2122 ± 0.0018, c1,? = 0.3319 ± 0.0054, and ?? =2.5915 ± 0.0024. The (b-y)? and m1,? colors obtained from absolute spectrophotometry of the Sunagree within 3-? with the solar colors derived here when thephotometric zero-points are determined from either the STIS HSTobservations of Vega or an ATLAS9 Vega model, but the c1,? and ?? synthetic colors inferred fromabsolute solar spectra agree with our solar colors only when thezero-points based on the ATLAS9 model are adopted. The Kurucz solarmodel provides a better fit to our observations than the MARCS model.For photometric values computed from the Kurucz models,(b-y)? and m1, ? are in excellentagreement with our solar colors independently of the adoptedzero-points, but for c1, ? and ??agreement is found only when adopting the ATLAS9 zero-points. Thec1, ? color computed from both the Kurucz and MARCSmodels is the most discrepant, probably revealing problems either withthe models or observations in the u band. The Teffcalibration of Alonso and collaborators has the poorest performance(~140 K off), while the relation of Casagrande and collaborators is themost accurate (within 10 K). We confirm that the Ramírez &Meléndez uvby metallicity calibration, recommended byÁrnadóttir and collaborators to obtain [Fe/H] in F, G, andK dwarfs, needs a small (~10%) zero-point correction to place the starsand the Sun on the same metallicity scale. Finally, we confirm that thec1 index in solar analogs has a strong metallicitysensitivity.Based on observations collected at the H. L. Johnson 1.5 m telescope atthe Observatorio Astronómico Nacional at San Pedro Mártir,Baja California, México.Tables 1-3 and 5 are only available inelectronic form at http://www.aanda.org
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