An updated theoretical scenario for globular cluster stars

In the rst part of this paper we revisit the history of theoretical predictions for HB luminosities in old Population II stellar clusters, starting from the results of \old" evolutionary computations to introduce in vari- ous steps all the available \new" physics. We discuss the influence of physical ingredients on selected evolutionary parameters, nally presenting models which incorporate all the most recent updating of the relevant physics. The evolutionary behavior of such models is extensively inves- tigated for selected choices about the cluster metallicity, discussing theoretical predictions concerning both cluster isochrones and the calibration of the parameterR in terms of the original amount of He in stellar matter. One nds that the \new" physics has a relevant influence on both these parameters, moving cluster ages into a much bet- ter agreement with current cosmological evaluations. This scenario is implemented by a further set of stellar models where element diusion is taken into account. The com- parison between theoretical scenarios with or without dif- fusion is presented and discussed. A discussion of current observational constraints in the light of the updated the- ory closes the paper.