THE EVOLUTION THROUGH H AND HE BURNING OF GALACTIC CLUSTER STARS

We extend previous evolutionary computations of intermediate-mass stars to lower masses in order to include the red giant branch (RGB) phase transition, i.e., the transition between models igniting He in a highly degenerate core and those igniting He quiescently. Selected stellar models down to 1 M. and having solar metallicity have been computed from the zero-age main sequence up to the onset of thermal pulses on the asymptotic giant branch. On this basis we produced theoretical isochrones and synthetic clusters in the range of ages 30 less-than-or-equal-to t less-than-or-equal-to 11,000 Myr, covering the range of age spanned by the large majority of Galactic clusters. We find that the variations among the relative frequencies of subgiants, red giants, and clump (central He-burning) stars are not strictly connected to the RGB phase transition. In particular we find that, although the RGB phase transition may be safely settled between 2 and 2.5 M., the mass limit discriminating between clusters dominated by clump stars and those dominated by subgiant stars is of the order of 1.8 M.. We note that very few stars (if any) are expected to populate the bright part of the RGB (i.e., above the clump luminosity) of any known Galactic open cluster. In addition, we find that in the range of ages between 0.6 and 10 Gyr, the luminosity of the central He-burning models is almost insensitive to the age, supporting the use of clump stars to derive the cluster distance moduli. The fit to the color-magnitude diagrams of a set of Galactic clusters, i.e., the Pleiades, Hyades, Praesepe, NGC 2420, and NGC 188, does show a good agreement between observational data and theoretical expectations. In particular, the canonical scenario appears able to account for both the observed location of cluster stars in the H-R diagram and for the observed relative abundances of stars in the various evolutionary phases. Therefore, we conclude that some periodically recurrent claims on the need for a core overshooting scenario, in order to account for the observed number of giant stars and for the turnoff morphology, turn out to be unfounded. Selected isochrones are finally presented in a form suitable both for a quick evaluation of the number ratio of stars in the various evolutionary phases and for comparisons with the observed color-magnitude diagrams.