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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.
| Accurate Coordinates and 2MASS Cross Identifications for (Almost) All Gliese Catalog Star
We provide precise J2000, epoch 2000 coordinates, andcross-identifications to sources in the 2MASS Point Source Catalog fornearly all stars in the Gliese, Gliese-Jahreiss, and Woolley catalogs ofnearby stars. The only Gliese objects where we were not successful aretwo Gliese sources that are actually QSOs; two proposed companions tobrighter stars, which we believe do not exist; four stars included inone of the catalogs but identified there as only optical companions; oneprobable plate flaw; and two stars that simply remain unrecovered. Forthe 4251 recovered stars, 2693 have coordinates based on Hipparcospositions, 1549 have coordinates based on 2MASS data, and 9 havepositions from other astrometric sources. All positions have beencalculated at epoch 2000 using proper motions from the literature, whichare also given here.
| Spectrophotometric properties of pre-main-sequence stars: the ?Chamaeleontis cluster
We present a study of flux-calibrated low-resolution opticalspectroscopy of 10 stars belonging to eight systems in the ~5-Myr-old?Chamaeleontis (?Cha) pre-main-sequence (PMS) starcluster. Using synthetic broad-band colours, narrow-band continuum,atomic and molecular lines derived from the spectra, we compare the?Cha stars to a slightly older PMS cluster, the ~8-Myr-old?Cha cluster and to main-sequence dwarfs. Using synthetic VRIcolours and other indices, we find that the relationship betweenbroad-band colours and spectroscopic temperature indicators for?Cha cluster members is indistinguishable from that of Gyr-olddwarfs. This result is identical to that found earlier in ?Cha.Gravity-sensitive line indices place the cluster between the giant anddwarf sequences, and there is clear evidence that ?Cha starshave lower surface gravity than ?Cha stars. This result isconsistent with ?Cha being the slightly younger PMS association,a few Myr younger according to the Hertzsprung-Russell (HR) diagramplacement of these two clusters and comparison with PMS evolutionarygrids. Late M-type ?Cha cluster members show a B-band fluxexcess of ~0.2mag compared to observations of standard dwarfs, whichmight be related to enhanced magnetic activity. A similar level ofexcess B-band emission appears to be a ubiquitous feature of low-massmembers of young stellar populations with ages less than a few hundredMyr, a very similar time-scale to the PMS phase of elevated relativeX-ray luminosity.
| BVRI photometry of the Gliese Catalogue stars
Photoelectri BVRI photometry on the Cousins (Kron-Cape) system has beenobtained for many of the southern faint stars in the Gliese Catalog(1969). This extends the work of Cousins (1980) and provides a uniformset of data for the nearby stars. Several red dwarfs are noted, whichwere used to define the red end of the Cousins system.
| UBVRI photoelectric photometry of nearby stars
In order to complete the photometric data of the Gliese (1969) 'Catalogof Nearby Stars', and in addition use these data for the Hipparcos spaceastrometry mission, program stars have been selected from the catalogand its supplements on the basis of their having an incomplete set ofUBVRI photometric data of magnitude lower than 13. The program developedrejects determinations of any magnitude or color index having a residualgreater than 2(sigma-prime), where sigma-prime is the standard deviationfor the determinations of unit weight.
| A Study of Low-Dispersion Spectra of M Stars in the Red Region.
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