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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.
| Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 Parsecs: The Northern Sample. I.
We have embarked on a project, under the aegis of the Nearby Stars(NStars)/Space Interferometry Mission Preparatory Science Program, toobtain spectra, spectral types, and, where feasible, basic physicalparameters for the 3600 dwarf and giant stars earlier than M0 within 40pc of the Sun. In this paper, we report on the results of this projectfor the first 664 stars in the northern hemisphere. These resultsinclude precise, homogeneous spectral types, basic physical parameters(including the effective temperature, surface gravity, and overallmetallicity [M/H]), and measures of the chromospheric activity of ourprogram stars. Observed and derived data presented in this paper arealso available on the project's Web site.
| New neighbours. V. 35 DENIS late-M dwarfs between 10 and 30 parsecs
This paper reports updated results on our systematic mining of the DENISdatabase for nearby very cool M-dwarfs (M 6V-M 8V, 2.0 <= I-J <=3.0, photometric distance within 30 pc), initiated by Phan-Bao et al.(\cite{phan-bao}, hereafter Paper I). We use M dwarfs with well measuredparallaxes (HIP, GCTP, ...) to calibrate the DENIS (MI, I-J)colour-luminosity relationship. The resulting distance error for singledwarfs is about 25%. Proper motions, as well as B and R magnitudes, weremeasured on archive Schmidt plates for those stars in the DENIS databasethat meet the photometric selection criteria. We then eliminate thegiants by a Reduced Proper Motion cutoff, which is significantly moreselective than a simple proper motion cutoff. It greatly reduces theselection bias against low tangential velocity stars, and results in anearly complete sample. Here we present new data for 62 red dwarfcandidates selected over 5700 square degrees in the DENIS database. 26of those originate in the 2100 square degrees analysed in Paper I, withimproved parameters here, and 36 were found in 3600 additional squaredegrees. 25 of those are new nearby dwarfs. We determine from thatsample of 62 stars a stellar density for 12.0 <= MI <=14.0 of /lineΦ{Icor}=(2.2 +/- 0.4)x 10-3 starspc-3 mag-1. This value is consistent withphotometric luminosity functions measured from deeper and smaller-fieldobservations, but not with the nearby star luminosity function. Inaddition we cross-identified the NLTT and DENIS catalogues to find 15similar stars, in parts of the sky not yet covered by thecolour-selected search. We present distance and luminosity estimates forthese 15 stars, 10 of which are newly recognized nearby dwarfs. Asimilar search in Paper I produced 4 red dwarf candidates, and we havethus up to now identified a total of 35 new nearby late-M dwarfs.
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
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