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Ýlgili Makaleler
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.
| Population studies. I - The Bidelman-MacConnell 'weak-metal' stars
BRVI and DDO photometry are presented for 309 Bidelman-MacConnell'weak-metal' stars. Radial velocities are calculated for most of thestars having Fe/H abundances of no more than -0.8. The photometricobservations were carried out using the 0.6-meter and 1.0-metertelescopes of the Siding Spring Observatory. Photometric taxonomy wasused to classify the stars as dwarfs, giants, red-horizontal branchstars, and ultraviolet-bright stars, respectively. It is found that 35percent of the stars are giants; 50 percent are dwarfs; and 5 percentbelong to the red-horizontal branch group. The role of selection effectsin investigations of the formation of the Galaxy is discussed on thebasis of the photometric observations and the observational constraintsproposed by Eggen et al. (1962).
| Southern metal-poor stars - UBVRI photometry
Considering the study of subdwarf kinematics and metallicities by Eggen,Lynden-Bell, and Sandage (1962), UBVRI photometry and normalizedultraviolet excesses are presented for 178 metal-poor stars, 144 ofwhich are contained in the kinematically unbiased list of Bidelman andMacConnell (1973). The Lowell 0.6 m telescope at Cerro Tololo was used,equipped with a single-channel photometer and a Ga-As photomultiplier.The final magnitudes and colors, number of observations, value ofdelta(U-B)0.6 (if B-V lies between 0.35 and 0.90), B and M class, andpublished spectral types for these stars are presented; severalextremely metal-poor stars are evident. In addition, sixteen nearbyvisual companions of the stars were measured, and their magnitudes andcolors are given.
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| Takýmyýldýz: | Ressam |
| Sað Açýklýk: | 05h05m32.27s |
| Yükselim: | -52°52'07.3" |
| Görünürdeki Parlaklýk: | 10.473 |
| özdevim Sað Açýklýk: | 0.7 |
| özdevim Yükselim: | -16.4 |
| B-T magnitude: | 11.125 |
| V-T magnitude: | 10.527 |
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