Basic physical parameters of a selected sample of evolved stars

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 the T eff values and the absolute V-band magnitude derived from Hipparcos parallaxes, are used to estimate basic stellar 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 excellent agreement (to within ∼0.05 mag) with the observed (B−V), confirming the reliability of the T eff −(B−V) 0 relation used in the isochrones. On the other hand, the estimated log g values are typically 0.2 dex lower than those derived from spectroscopy; this effect has a negligible impact on [Fe/H] determinations. The estimated diameters θ have been compared with limb darkening-corrected ones measured with independent methods, finding an agreement better than 0.3 mas within the 1 < θ < 10 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 been derived from observations of field giants rather than from open clusters and field dwarfs and subdwarfs. The age-metallicity relation is characterized by close-to-solar metallicities 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 of young stars (less than 1 Gyr) is comparable to the observational errors, indicating that stars in the solar neighbourhood are formed from interstellar matter of quite homogeneous chemical composition. The three giants of our sample which have been proposed to host planets are 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 so we may be sampling a different stellar population from most radial velocity searches for extrasolar planets. We also confirm the previous indication that the radial velocity variability tends to increase along the RGB, and in particular with the stellar radius.

Chemical composition of AY Ceti: A flaring, spotted star with a white dwarf companion

“…Identical [Fe/H] = −0.33 for AY Cet have been determined also in the recent studies of Biazzo et al (2007) and da Silva et al (2006). Lower values of the metallicity were determined in the earlier studies of Randich et al (-0.5 dex, 1993) and Berdyugina (-0.58 dex, 1994).…”

“…Simon et al determined that the radius of the AY Cet B white dwarf is r 2 = 0.012 R ⊙ , and the mass m 2 = 0.55 M ⊙ , the radius of the primary AY Cet A is r 1 = 6.8 R ⊙ , and the mass m 1 = 2.09 M ⊙ . Mass and radius of AY Cet were also determined in several later studies: r 1 = 6.6 R ⊙ by Schrijver & Zwaan (1991); r 1 = 8.18 R ⊙ and m 1 = 1.95 M ⊙ by da Silva et al (2006); and r 1 = 7.9 R ⊙ and m 1 = 2.0 M ⊙ by Massarotti et al (2008).…”

Seismic analysis of HD 43587Aa, a solar-like oscillator in a multiple system

“…Another minimization performed using the Asteroseismic modelling Portal (AMP, Metcalfe et al 2012b) led to an age of 5.66 Gyr, while the other parameters (mass, radius, temperature, luminosity) were found to be very close to those coming from the present minimization. Morel et al (2013) reported another estimate of the age, 4.94 ± 0.53 Gyr, obtained with the PARAM isochrone inversion tool (Girardi et al 2000;da Silva et al 2006). It is worth pointing out that this age is based solely on grids of stellar evolutionary tracks without considering any asteroseismic constraints.…”

The CORALIE survey for southern extrasolar planets

“…Metallicities, together with the effective temperatures and absolute V-band magnitudes are used to estimate basic stellar parameters (ages, masses, radii and log g) using theoretical isochrones from Girardi et al (2000) and a Bayesian estimation method described in da Silva et al (2006) 1 . Resulting stellar parameters are listed in Table 1.…”