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Deriving temperature, mass, and age of evolved stars from high-resolution spectra. Application to field stars and the open cluster IC 4651
Aims.We test our capability of deriving stellar physical parameters ofgiant stars by analysing a sample of field stars and the well studiedopen cluster IC 4651 with different spectroscopic methods. Methods: Theuse of a technique based on line-depth ratios (LDRs) allows us todetermine with high precision the effective temperature of the stars andto compare the results with those obtained with a classical LTEabundance analysis. Results: (i) For the field stars we find that thetemperatures derived by means of the LDR method are in excellentagreement with those found by the spectral synthesis. This result isextremely encouraging because it shows that spectra can be used tofirmly derive population characteristics (e.g., mass and age) of theobserved stars. (ii) For the IC 4651 stars we use the determinedeffective temperature to derive the following results. a) The reddeningE(B-V) of the cluster is 0.12±0.02, largely independent of thecolor-temperature calibration used. b) The age of the cluster is1.2±0.2 Gyr. c) The typical mass of the analysed giant stars is2.0±0.2~Mȯ. Moreover, we find a systematicdifference of about 0.2 dex in log g between spectroscopic andevolutionary values. Conclusions: We conclude that, in spite of knownlimitations, a classical spectroscopic analysis of giant stars mayindeed result in very reliable stellar parameters. We caution that thequality of the agreement, on the other hand, depends on the details ofthe adopted spectroscopic analysis.Based on observations collected at the ESO telescopes at the Paranal andLa Silla Observatories, Chile.
| Basic physical parameters of a selected sample of evolved stars
We present the detailed spectroscopic analysis of 72 evolved stars,which were previously studied for accurate radial velocity variations.Using one Hyades giant and another well studied star as the referenceabundance, we determine the [Fe/H] for the whole sample. Thesemetallicities, together with the T_eff values and the absolute V-bandmagnitude derived from Hipparcos parallaxes, are used to estimate basicstellar parameters (ages, masses, radii, (B-V)0 and log g)using theoretical isochrones and a Bayesian estimation method. The(B-V)0 values so estimated turn out to be in excellentagreement (to within ~0.05 mag) with the observed (B-V), confirming thereliability of the T_eff-(B-V)0 relation used in theisochrones. On the other hand, the estimated log g values are typically0.2 dex lower than those derived from spectroscopy; this effect has anegligible impact on [Fe/H] determinations. The estimated diametersθ have been compared with limb darkening-corrected ones measuredwith independent methods, finding an agreement better than 0.3 maswithin the 1<θ<10 mas interval (or, alternatively, findingmean differences of just 6%). We derive the age-metallicity relation forthe solar neighborhood; for the first time to our knowledge, such arelation has been derived from observations of field giants rather thanfrom open clusters and field dwarfs and subdwarfs. The age-metallicityrelation is characterized by close-to-solar metallicities for starsyounger than ~4 Gyr, and by a large [Fe/H] spread with a trend towardslower metallicities for higher ages. In disagreement with other studies,we find that the [Fe/H] dispersion of young stars (less than 1 Gyr) iscomparable to the observational errors, indicating that stars in thesolar neighbourhood are formed from interstellar matter of quitehomogeneous chemical composition. The three giants of our sample whichhave been proposed to host planets are not metal rich; this result is atodds with those for main sequence stars. However, two of these starshave masses much larger than a solar mass so we may be sampling adifferent stellar population from most radial velocity searches forextrasolar planets. We also confirm the previous indication that theradial velocity variability tends to increase along the RGB, and inparticular with the stellar radius.
| Hyades and Sirius supercluster members brighter than magnitude (V) 7.1. II - Right ascension six to twelve hours
The present star sample is contained in the Bright Star Catalogue andits Supplement, augmented with a further supplement of 788 stars foundduring various observing programs over the past 40 years. Accurate,four-color and H-beta, or (RI), photometry is available for most of thesupercluster members. The criteria for membership are the comparisons ofthe proper motion, radial velocity, and luminosity obtained from thesupercluster parameters with the observed motions and the luminosityderived from the photometric parameters. New proper motions, based onall available catalogs, have been derived for the additional 788 starsdiscussed here, as well as all supercluster members.
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