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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.
| X-ray observations of NGC 3077
ROSAT X-ray observations show that the irregular starburst galaxy NGC3077 has an X-ray luminosity of 2.1 x 1038 erg/s centered onthe inner 1' nuclear region, which is consistent with that expected fromother well observed starbursts (e.g. M82). The X-ray emission can not beaccounted for by a point-like nucleus but can be modeled by a hotinterstellar medium (ISM) with kT = 0.65 +/- 0.51 keV and a centralhydrogen number density about 0.75/cu.cm. Alternative interpretationssuch as X-rays binaries would have to have a steep X-ray luminosityfunction to explain the non-detection of bright X-ray binaries, while atthe same time, young stars would have to contribute X-rays 7 times lessthan observed. If one assumes an ISM origin, the X-ray cooled mass 4.5 x107 solar mass and the cooling rate 0.091 solar mass/yr arecomparable to both the molecular mass 2 x 107 solar mass ofthe three giant molecular complexes detected in the same region and thestar formation rate 0.1 solar mass/yr determined from H alpha andinfrared observations. Both the high formation rates of molecular cloudsand stars in NGC 3077, as well as in M82 in the same M81 group, may bedue to tidal forces or ejection related to closer proximity to M81 about5 x 108 years ago. NGC 3077 and M82 are linked to the nearbyM81 by a neutral hydrogen bridge. We find that there is an excess ofnumber of X-ray sources over the X-ray background within the inner 10'region of the Position Sensitive Proportional Counter (PSPC) center thatis statistically significant at the 99.8% level Some sources aresituated close to the hydrogen bridge and thus may belong to the group.Possible origins of these sources are discussed.
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