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Basic physical parameters of a selected sample of evolved stars

¹ Observatório Nacional - MCT, Rio da Janeiro, Brazil e-mail: licio@on.br
² Osservatorio Astronomico di Padova - INAF, Padova, Italy
³ European Southern Observatory, Garching bei München, Germany
⁴ Max-Planck-Institut für Astronomie, Heidelberg, Germany
⁵ Kiepenheuer-Institut für Sonnenphysik, Freiburg, Germany
⁶ Departamento de Física - UFRN, Natal, Brazil
⁷ Thüringer Landessternwarte, Tautenburg, Germany
⁸ Max-Planck-Institut für Astrophysik, Garching bei München, Germany

Received: January 1900
Accepted: 11 July 2006

Abstract

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 reference abundance, we determine the [Fe/H] for the whole sample.These metallicities, together with theT_eff values and the absoluteV-band magnitude derived from Hipparcos parallaxes, are used to estimatebasic stellar parameters (ages, masses, radii, and)using theoretical isochrones and a Bayesian estimation method.The values so estimated turn out to be in excellent agreement(to within ~0.05 mag) with the observed, confirming thereliability of theT_eff- relation used in the isochrones.On the other hand, the estimated values are typically0.2 dex lower than those derived from spectroscopy;this effect has a negligible impact on [Fe/H] determinations.The estimated diametersθ have been compared with limb darkening-corrected onesmeasured with independent methods, finding an agreement betterthan 0.3 mas within the mas interval (or, alternatively,finding mean differences of just 6%).We derive the age-metallicity relation for the solar neighborhood;for the first time to our knowledge, such a relation has beenderived from observations of field giants rather than fromopen clusters and field dwarfs and subdwarfs.The age-metallicity relation is characterized by close-to-solarmetallicities for stars younger than ~4 Gyr, and by a large[Fe/H] spread with a trend towards lower metallicities for higher ages.In disagreement with other studies, we find that the [Fe/H] dispersion ofyoung stars (less than 1 Gyr) is comparable to the observational errors,indicating that stars in the solar neighbourhood are formed frominterstellar matter of quite homogeneous chemical composition.The three giants of our sample which have been proposed to host planetsare not metal rich; this result is at odds with those for main sequence stars.However, two of these stars have masses much larger than a solar mass sowe may be sampling a different stellar population from most radial velocitysearches for extrasolar planets.We also confirm the previous indication that the radial velocityvariability tends to increase along the RGB, and in particularwith the stellar radius.