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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.
| Rotational and Radial Velocities for a Sample of 761 HIPPARCOS Giants and the Role of Binarity
We present rotational and radial velocities for a sample of 761 giantsselected from the Hipparcos Catalogue to lie within 100 pc of the Sun.Our original goal was to examine stellar rotation in field giants usingspectroscopic line broadening to look for evidence of excess rotationthat could be attributed to planets that were engulfed as the parentstars expanded. Thus we were obliged to investigate other sources ofline broadening, including tidal coupling in close binaries andmacroturbulence. For all the binaries in our sample with periods shorterthan 20 days the orbits have been circularized, while about half theorbits with periods in the range 20-100 days still show significanteccentricity. All our primaries in orbits shorter than 30 days show linebroadening consistent with synchronized rotation, while about half theprimaries with periods in the range 30-120 days are synchronized. Tostudy the dependence of rotation on stellar evolution when tidal effectsare not important, we used a subsample of single stars and members inwide binaries. We found evidence to suggest that the first dredge-up mayplay a role in speeding up the rotation of the observable outer layersof giants and that the rotational velocity of horizontal branch stars islarger by a few km s-1 than that of first-ascent giants withsimilar mass, effective temperature, and radius. Finally, we found threegiants that rotate more rapidly than expected. We conjecture that theyacquired their excess angular momentum by ingesting planets.Some of the results presented here used observations made with theMultiple Mirror Telescope, a joint facility of the SmithsonianInstitution and the University of Arizona.
| Stars with large proper motions in the astrographic zones +32° and +33° (List II)
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