AN EVOLUTIONARY SCENARIO FOR PRIMEVAL STELLAR POPULATIONS

We present a grid of evolutionary computations of stellar structures w ith an original amount of heavy elements Z = 10(-10) and for an origin al He abundance Y = 0.23. The range of stellar masses 0.7 less-than-or -equal-to M less-than-or-equal-to M. has been investigated, following the evolution from the initial ZAMS structures till the He flash or, f or larger masses, up to the final cooling of the C-O degenerate cores or up to the C ignition. Parallel computations are presented for Z = 1 0(-6) and Z = 10(-4), allowing the investigation of the evolutionary b ehavior of stellar structures when lowering the metallicity from a nor mal Population II composition down to the expected cosmological abunda nce of heavy elements. We show that (1) the evolution of the stellar i nteriors appears unaffected by recent improvements in the evaluation o f the opacity, at least below Z almost-equal-to 10(-3), and (2) the in fluence of metallicity on the stellar opacity is vanishing for Z value s lower than almost-equal-to 10(-5). However. we find that the influen ce of Z on the efficiency of CNO H burning remains important down to, at the least, Z almost-equal-to 10(-6). Selected evolutionary paramete rs are discussed, investigating the dependence on Z of both the critic al mass for the red giant branch transition M(Fl) and the upper mass l imit for carbon deflagration M(up). On this basis. the color evolution of a primeval stellar populations and the occurrence of carbon deflag rating stars are finally shortly discussed. In the Appendix, analytica l relations are presented giving the dependence of selected evolutiona ry parameters on star masses and metallicities.