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מאמרים קשורים
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.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The Tokyo PMC catalog 90-93: Catalog of positions of 6649 stars observed in 1990 through 1993 with Tokyo photoelectric meridian circle
The sixth annual catalog of the Tokyo Photoelectric Meridian Circle(PMC) is presented for 6649 stars which were observed at least two timesin January 1990 through March 1993. The mean positions of the starsobserved are given in the catalog at the corresponding mean epochs ofobservations of individual stars. The coordinates of the catalog arebased on the FK5 system, and referred to the equinox and equator ofJ2000.0. The mean local deviations of the observed positions from theFK5 catalog positions are constructed for the basic FK5 stars to comparewith those of the Tokyo PMC Catalog 89 and preliminary Hipparcos resultsof H30.
| A photometric and radial-velocity analysis of the intermediate-age open cluster NGC 752
Using all available proper-motion and radial-velocity data, includingnew radial-velocity observations obtained for this investigation,probable members of the open cluster NGC 752 have been identified.Photoelectric data on six systems have been transformed and collated toform an internally consistent sample on the BV system. Binaries havebeen identified using photometric and radial-velocity data, including aphotographic survey for variability and the radial-velocity observationsof this study. Analysis of the data leads to the following clusterparameters and their probable errors: E(B - V) = 0.035 plus or minus0.005 mag, Fe/H = -0.15 plus or minus 0.05 dex, and (m - M) = 8.25 plusor minus 0.10 mag. The spread in color among stars in thecolor-magnitude diagram (cmd) along the main sequence from the turnoffto the unevolved main sequence is the consequence of a rich populationof binaries. Due to its age and the comprehensive data available for thecluster, NGC 752 provides an ideal test of a variety of evolutionaryphenomena. Comparison with theoretical isochrones normalized in aninternally consistent manner leads to the conclusion that the morphologyand distribution of stars in the cmd can best be matched using modelsthat include convective overshoot, particularly those of Schaller et al.(1992, A&AS, 96, 269). Despite their differences, the traditional andthe overshoot isochrones both imply very similar ages, 1.9 plus or minus0.2 Gyr and 1.7 plus or minus 0.1 Gyr, respectively, for the cluster.The Li abundances for the giants confirm that the giant branch isdominated by clump stars and first-ascent giants below the luminosity ofthe clump. The position and size of the Li dip among the main-sequencestars, compared to the Hyades, is readily explained by stellar evolutionwith convective overshoot. It is predicted that among turnoff stars inthe intermediate-age range Li will cease to be a unique function of ageat a given color. Chromospheric flux is shown to be a well-definedfunction of color for single, unevolved stars, identical to that foundfor the Hyades, and the relation for NGC 752 falls within theVaughan-Preston gap. However, the slope of the relation requires thatincreasing color implies increasing age for the bluer portion of theweak-emission boundary. The combined effect of small samples, randomerrors, emission limits, a possible selection bias in favor of turnoffstars, and metallicity corrections is to render highly questionable anyinterpretation of time-variable star formation within the Galaxy basedupon chromospheric ages.
| The studies of proper motions in the regions of open clusters. II - NGC 752
Relative proper motions of 1777 stars in the region of the open clusterNGC 752 have been determined from the PDS measurements of eleven Pulkovonormal astrograph plates. In a 110-arcmin-diameter circle around thecluster, 175 stars have membership probabilities P greater than 0percent. The total number of cluster stars, obtained by weighting themwith membership probabilities, reaches 130. The catalog of propermotions is complete down to m(pg) = 150 mag, the limiting value is 154mag. The average standard error of the catalog proper motions variesfrom + or - 000075 arcsec/yr to + or - 00011 arcsec/yr for the rest. Theluminosity function for cluster stars shows a deficiency of faint stars.
| Vitesses radiales dans la region de l'amas NGC 752.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1970A&A.....4..404R&db_key=AST
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| קבוצת-כוכבים: | אנדרומדה |
| התרוממות ימנית: | 01h54m53.04s |
| סירוב: | +38°17'07.5" |
| גודל גלוי: | 8.531 |
| מרחק: | 238.663 פארסק |
| תנועה נכונה: | 5.5 |
| תנועה נכונה: | -1.7 |
| B-T magnitude: | 9.762 |
| V-T magnitude: | 8.633 |
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