Theoretical models for helium-burning stars in intermediate-metallicity globular clusters

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Abstract

We present a set of evolutionary tracks covering the whole phase of He burning (during both horizontal.. branch [HB] and asymptotic giant branch [AGB] evolution) for stellar models, intended to represent evolving stars in intermediate-metallicity Galactic globular clusters. Structural parameters for the initial models have been derived from evolutionary computations of suitable H-burning progenitors. Excellent agreement has been found with previous evaluations given in the literature concerning both the mass of the H-exhausted core at the He flash and the amount of helium driven by convection in the surface layers. Models starting their HB evolution at various effective temperatures have been followed until the onset r thermal pulses in an advanced phase of their AGB evolution. Extensive tables are presented for selected evolutionary parameters, allowing a discussion of evolutionary features. The occurrence of a three-component (He, C, and 0) mixture in the central core has been taken into account following previous prescriptions. Theoretical evidences for an "early HB" phase preceding the zero-age horizontal branch are recalled and briefly dis cussed. The results support a previous suggestion for a negligible efficiency of the "breathing pulses" phenomenon. The general evolutionary scenario for the HB and the AGB is recalled and discussed in connection with current observational parameters for globular cluster stars.

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