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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.
| Multicolor Photometric Observations of Candidate Optical Counterparts to ROSAT Faint X-Ray Sources in a 1 Square Degree Field of the BATC Survey
We present optical candidates for 75 X-ray sources in a ~1deg2 overlapping region with the 1997 medium-deep ROSATsurvey by Molthagen et al. These candidates are selected using themulticolor CCD imaging observations made for the T329 field of theBeijing-Arizona-Taiwan-Connecticut (BATC) Sky Survey, which uses theNAOC 0.6/0.9 m Schmidt telescope with 15 intermediate-band filterscovering the wavelength range 3360-9745 Å. These X-ray sources arerelatively faint (CR<<0.2 s-1) and thus are mostly notincluded in the ROSAT Bright Source Catalogue; they also remain as X-raysources without optical candidates in a previous identification programcarried out by the Hamburg Quasar Survey. Within their position errorcircles, almost all of the X-ray sources are observed to contain one ormore spatially associated optical candidates down to the magnitudemV~23.1. We have classified 149 of 156 detected opticalcandidates with 73 of the 75 X-ray sources with a new method thatpredicts a redshift for nonstellar objects, which we have termed theSED-based Object Classification Approach. These optical candidatesinclude 31 QSOs, 39 stars, 37 starburst galaxies, 42 galaxies, and seven``just''-visible objects. Twenty-eight X-ray error circles have only onevisible object in them: nine QSOs, three normal galaxies, eightstarburst galaxies, six stars, and two of the just-visible objects. Wehave also cross-correlated the positions of these optical objects withNASA Extragalactic Database, the FIRST radio source catalog, and the TwoMicron All Sky Survey. Separately, we have also SED-classified theremaining 6011 objects in our field of view. Optical objects are foundat the 6.5 σ level above what one would expect from a randomdistribution; only QSOs are overrepresented in these error circles atgreater than 4 σ frequency. We estimate redshifts for allextragalactic objects and find a good correspondence between ourpredicted redshift and the measured redshift (a mean error of 0.04 inΔz). There appears to be a supercluster at z~0.3-0.35 in thisdirection, including many of the galaxies in the X-ray error circlesthat are found in this redshift range.
| Multiwavelength observations of the field HS 47.5/22 in Ursa Major. I. The X-ray catalogue of a medium deep ROSAT survey
We present the X-ray source catalogue obtained from a ROSAT survey inthe HQS field HS 47.5/22. The survey consists of 48 overlapping PSPCpointings which were first analysed individually, and then merged toincrease the sensitivity. Both modes bring forth sources missed in theother, showing that both are necessary to detect all X-ray sources inthe field. The final catalogue contains 574 X-ray sources. Theidentifications are mainly based on the HQS objective prism plate whichallows object classification down to a magnitude m_b^{0.4 < -0.4ex} ~18.5', and a rough distinction between red and blue objects more than amagnitude deeper. Follow-up observations were performed for a number ofobjects, mainly faint quasar candidates. Partly based on observationsfrom the German-Spanish Astronomical Center, Calar Alto, operated by theMax-Planck-Institut fur Astronomie, Heidelberg, jointly with the SpanishNational Commission for Astronomy. Tables 4 to 6 are only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html. Figure 7is published electronically and is made available athttp://www.ed-phys.fr/Abstract.html
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