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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.
| Optical Polarization Mapping Toward the Interface Between the Local Cavity and Loop I
The Sun is located inside an extremely low density and quite irregularvolume of the interstellar medium, known as the Local Cavity (LC). Ithas been widely believed that some kind of interaction could beoccurring between the LC and Loop I, a nearby superbubble seen in thedirection of the Galactic center. As a result of such interaction, awall of neutral and dense material, surrounded by a ring-shaped feature,would be formed at the interaction zone. Evidence of this structure waspreviously observed by analyzing the soft X-ray emission in thedirection of Loop I. Our goal is to investigate the distance of theproposed annular region and map the geometry of the Galactic magneticfield in these directions. On that account, we have conducted an opticalpolarization survey of 878 stars from the Hipparcos catalog. Our resultssuggest that the structure is highly twisted and fragmented, showingvery discrepant distances along the annular region: ?100 pc on theleft side and 250 pc on the right side, independently confirming theindication from a previous photometric analysis. In addition, thepolarization vectors' orientation pattern along the ring also shows awidely different behavior toward both sides of the studied feature,running parallel to the ring contour on the left side and showing norelation to its direction on the right side. Altogether, these evidencessuggest a highly irregular nature, casting some doubt on the existenceof a unique large-scale ring-like structure.
| The PASTEL catalogue of stellar parameters
Aims: The PASTEL catalogue is an update of the [Fe/H] catalogue,published in 1997 and 2001. It is a bibliographical compilation ofstellar atmospheric parameters providing (T_eff, log g, [Fe/H])determinations obtained from the analysis of high resolution, highsignal-to-noise spectra, carried out with model atmospheres. PASTEL alsoprovides determinations of the one parameter T_eff based on variousmethods. It is aimed in the future to provide also homogenizedatmospheric parameters and elemental abundances, radial and rotationalvelocities. A web interface has been created to query the catalogue onelaborated criteria. PASTEL is also distributed through the CDS databaseand VizieR. Methods: To make it as complete as possible, the mainjournals have been surveyed, as well as the CDS database, to findrelevant publications. The catalogue is regularly updated with newdeterminations found in the literature. Results: As of Febuary2010, PASTEL includes 30151 determinations of either T_eff or (T_eff,log g, [Fe/H]) for 16 649 different stars corresponding to 865bibliographical references. Nearly 6000 stars have a determination ofthe three parameters (T_eff, log g, [Fe/H]) with a high qualityspectroscopic metallicity.The catalogue can be queried through a dedicated web interface at http://pastel.obs.u-bordeaux1.fr/.It is also available in electronic form at the Centre de DonnéesStellaires in Strasbourg (http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=B/pastel),at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/515/A111
| A Survey for A Coeval, Comoving Group Associated with HD 141569
We present the results of a search for a young stellar moving groupassociated with the star HD 141569—a nearby, isolated Herbig AeBeprimary member of a 5 ± 3 Myr-old triple star system on theoutskirts of the Sco-Cen complex. Our spectroscopic survey identified apopulation of 21 Li-rich, lsim 30 Myr-old stars within 30° of HD141569 which possess similar proper motions with the star. The spatialdistribution of these Li-rich stars, however, is not suggestive of amoving group associated with the HD 141569 triplet, but rather thissample appears cospatial with Upper Scorpius (US) and Upper CentaurusLupus (UCL). We apply a modified moving cluster parallax method tocompare the kinematics of these youthful stars with those of the US andUCL. Eight new potential members of US and five new potential members ofUCL are identified. A substantial moving group with an identifiablenucleus within 15° (~ 30 pc) of HD 141569 is not found in thissample. Evidently, the HD 141569 system formed ~ 5 Myr ago in relativeisolation, tens of parsecs away from the recent sites of star formationin the Ophiucus-Scorpius-Centaurus region.
| Effective temperature scale and bolometric corrections from 2MASS photometry
We present a method to determine effective temperatures, angularsemi-diameters and bolometric corrections for population I and II FGKtype stars based on V and 2MASS IR photometry. Accurate calibration isaccomplished by using a sample of solar analogues, whose averagetemperature is assumed to be equal to the solar effective temperature of5777 K. By taking into account all possible sources of error we estimateassociated uncertainties to better than 1% in effective temperature andin the range 1.0-2.5% in angular semi-diameter for unreddened stars.Comparison of our new temperatures with other determinations extractedfrom the literature indicates, in general, remarkably good agreement.These results suggest that the effective temperaure scale of FGK starsis currently established with an accuracy better than 0.5%-1%. Theapplication of the method to a sample of 10 999 dwarfs in the Hipparcoscatalogue allows us to define temperature and bolometric correction (Kband) calibrations as a function of (V-K), [m/H] and log g. Bolometriccorrections in the V and K bands as a function of T_eff, [m/H] and log gare also given. We provide effective temperatures, angularsemi-diameters, radii and bolometric corrections in the V and K bandsfor the 10 999 FGK stars in our sample with the correspondinguncertainties.
| UVBY observations of A, F, G and K field stars
Photoelectric data in the uvby system have been obtained for about 800southern stars of the Hipparcos Input Catalog (Grenon, 1982, 1985). Mostof the stars are F and G main sequence and fall in the magnitude range V= 8-11.
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| קבוצת-כוכבים: | נחש |
| התרוממות ימנית: | 15h27m42.64s |
| סירוב: | -02°45'18.6" |
| גודל גלוי: | 10.569 |
| תנועה נכונה: | -28.8 |
| תנועה נכונה: | -19.4 |
| B-T magnitude: | 11.299 |
| V-T magnitude: | 10.63 |
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