Evolutionary sequences of stellar models with new radiative opacities. IV. Z=0.004 and Z=0.008

We present detailed tabulations of two large grids of stellar models with the initial chemical compositions [Y=0.240, Z=0.004] and [Y=0.250, Z=0.008], computed with the most recent radiative opacities (OPAL) by Iglesias et al. (1992). This paper continues the series initiated by Bressan et al. (1993) for the grid with [Y=0.280, Z=0.020], the reference solar like abundances, followed by Fagotto et al. (1994a) for the grids with compositions [Y=0.230, Z=0.0004] and [Y=0.352, Z=0.05]. The tracks span a wide range of initial masses from 0.6 M. to 120 M. and extend from the zero age main sequence (ZAMS) till very advanced evolutionary phases. Specifically, low- and intermediate-mass stars are followed till the beginning of the thermally pulsing regime of the asymptotic giant branch phase (TP-AGB), while massive stars axe followed till the core C-ignition. The models of low- and intermediate-mass stars are calculated at constant mass, whereas those of massive stars are followed in presence of mass loss by stellar wind incorporating a suitable dependence on the metallicity. The results for all the models are given in extensive tables which summarize also the lifetimes of the various phases and the variations of the surface abundances by dredge-up phenomena and mass loss by stellar wind. The salient features brought by the different metallicity and helium content are briefly outlined. In virtue of their large coverage of masses, evolutionary phases, and chemical compositions, such grids of evolutionary tracks are basic to studies of population synthesis. In particular they are suited to interpret the CMDs of metal-rich globular clusters and of the typical stellar content of the Large Magellanic Cloud.