Helium abundance, age and mixing length parameter for low mass stars
Abstract - Calibration of nearby visual binary stars on the HR diagram is used to determine the helium abundance, age and mixing length parameter for stars other than the Sun. Four Population I low mass systems with high quality data are analysed by means of modern evolutionary stellar models : Eta-Cassiopeiae, 70 Ophiuchi, Xi-Bootis and 85 Pegasi. Complementing these results by those for the Sun and it is shown that in the framework of the mixing length formalism to describe convection, a unique value of the mixing length parameter, equal to the solar one can be used to model these objects. Except for 85 Pegasi system, which cannot be explained by means of standard stellar evolutionary models, the helium abundance is determined with a precision of 0.02 (abundance in mass) and the age with a precision of 2 Gyr. A concomitant positive relation between metallicity and helium abundance is found for these stars, corresponding to a mean value of about dy/dz ~ 3 +/- 2 (relative helium to heavier elements enrichment parameter) but no clear correlation between age and metallicity exists.The consequences on the results of the use of the new parallaxes from HIPPARCOS mission are briefly discussed.