The HR Diagram for Late-Type Nearby Stars as a Function of Helium and Metallicity

Recent theoretical stellar models are used to discuss the helium abundance of a number of low-mass stars for which the position in the Hertzsprung-Russell diagram and the metallicity are known with high accuracy. Hipparcos has provided very high quality parallaxes of a sample of a hundred disk stars, of type F to K, located in the solar neighbourhood. Among these stars we have carefully selected those for which detailed spectroscopic analysis has provided e ective temperature and [Fe/H] ratio with a high accuracy. We have calculated evolved stellar models and their associated isochrones in a large range of mass, for several values of the metallicity and of the helium abundance and we took into account an -element enrichment in the metal-de cient stars. The input physics is recent and appropriate to the considered stellar mass range. We discuss the position in the H-R diagram of those stars which can be considered as non-evolved. Once the physics of the models has been xed, this position only depends on metallicity and helium abundance. We nd that the thickness of the observational main sequence is of about 0.25 magnitude, for stars spanning a metallicity range from [Fe/H] = 1 to +0:2, while theoretical stellar models predict a width of about 0.45 magnitude. The position in the H-R diagram of stars of solar metallicity or close to it is well accounted for by theoretical stellar models. Problems arise with the moderately metal de cient stars which lie quite close to the stars of solar metallicity and very far from the theoretical isochrones corresponding to their expected chemical composition. To reconcile theory and observations very low values of the helium abundance, well below the primordial helium abundance, would be needed. We brie y discuss the possible reasons of this discrepancy: improvements to bring to the physics of the models, inaccuracies of observations.