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Astron. Astrophys. 350, 587-597 (1999)

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1. Introduction

Valuable tests of the stellar evolution theory and strong constraints on the physical description of stellar interiors are mainly provided by the study of the most-accurately observed stellar objects. The number of observable parameters accessible through observations is important as well as the accuracy on their determination. The Sun, the nearby stars including members of visual binary systems, and stars belonging to open clusters represent some of these best-known objects.

The observations of nearby low-mass stars have been greatly improved, and their number has substantially increased, during the last few years. Distances have been determined with a very high precision with the Hipparcos satellite. In parallel, ground-based measurements have provided high resolution spectra and multicolour photometry of an appreciable number of stars of the solar neighbourhood. On the other hand, models of stellar atmospheres have largely benefited from progresses in the theoretical description of microscopic physics, in particular opacities. The analysis of the observational data, using model atmospheres, has provided bolometric magnitudes, effective temperatures and abundances with an accuracy which had never been reached previously. In addition, several nearby stars are members of visual binary systems and their mass is known.

The study of stars of low mass has many important physical and astrophysical implications. First, several uncertainties remain on the physics of their internal structure. For instance, the imperfect understanding of the convective transport or of the non local thermodynamical equilibrium (non-LTE) effects in atmospheres and envelopes have important consequences: atmospheres and envelopes serve as external boundary conditions for the interior models, fixing the radius, and are also used in the analysis of observational data to determine abundances, effective temperatures and gravities. On the other hand, transport processes at work in the deep interior need to be better constrained; they link the convective zone to deep nuclear burning regions, involving consequences for the surface abundances (lithium and light elements but also heavy elements), and for the estimated age and global parameters, when fresh helium is brought to the burning core.

An important question is also that of helium content, not measurable in the photosphere due to the lack of lines in the spectrum. The initial helium content of a star determines its lifetime and internal structure and is also a witness of past galactic history. The knowledge of the initial helium abundance of stars born in different sites with different metallicities is therefore fundamental for studies of the chemical evolution of the Galaxy.

We study here a sample of a hundred nearby disk stars, of spectral types in the range F to late K, by means of theoretical stellar models. Our aim is to discuss the ability of the models to reproduce high quality observations and, when possible, to determine the helium content of the stars.

In Sect. 2 we make a rapid summary of some results previously obtained on the subject, from studies of a few particularly well-known objects. In Sect. 3 we carefully examine the observational data available for nearby stars in order to extract subsamples corresponding to the very best accuracies. We then present the derived Hertzsprung-Russell (HR) diagrams of stars of the solar neighbourhood which have the smallest error bars ever obtained. Sect. 4 describes the theoretical models used to compute isochrones for given chemical compositions. In Sect. 5 we show evidence for discrepancies between observations and theoretical models, which are particularly obvious for stars with metallicities below [Fe/H] =-0.5. In Sect. 6 we examine this problem in the light of recent works on departure from LTE for iron (Thévenin & Idiart 1999) and on microscopic diffusion of helium and heavier elements in metal-poor stars (Morel & Baglin 1999). We show that the cumulative effects of these two processes is large enough to remove the discrepancy shown in Sect. 5. Sect. 7 considers the case of binary stars with known dynamical masses. Sect. 8 addresses the question of the helium content of individual objects of the sample. Sect. 9 summarizes our conclusions.

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© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999
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