OXYGEN-ENHANCED MODELS FOR GLOBULAR-CLUSTER STARS .1. EVOLUTIONARY TRACKS

Evolutionary tracks for 13-16 mass values (depending on the assumed chemical composition) in the range 0.15 less-than-or-equal-to M/M . less-than-or-equal-to 1.25 have been computed for each of 10 metallicities between [Fe/H] = -2.26 and [Fe/H] = -0.47. The helium content was assumed to vary from Y = 0.2350 to 0.2454 over this range in [Fe/H]. Oxygen overabundances were chosen to obey [O/Fe] = -0.5[Fe/H], for [Fe/H] greater-than-or-equal-to -1.0, and [O/Fe] = -0.2[Fe/H] + 0.3, for lower iron abundances, based on an analysis of field star data. Solar number abundance ratios were adopted for all of the other heavy elements. The tracks were extended either to an age of 30 Gyr or, in the case of models with masses greater-than-or-equal-to 0.7 M ., to the tip of the red giant branch. Altogether, 46 tracks were evolved to the helium flash point. Particular care was taken to normalize the T(eff) and B-V scales of the models as accurately as possible to available observational constraints. This involved the setting up of a semiempirical set of color transformations (and bolometric corrections) that is tabulated herein for use by other workers. The properties of selected models along the entire evolutionary sequences for 0.8 M . (or 0.85 M ., in the case of the three highest [Fe/H] values) are tabulated in detail. In addition, for all of the computed giant branch tracks, such information as the luminosity and core mass at the helium flash, the final C-12/C-13 and C/N number abundance ratios obtained on completion of the first dredge-up phase, and the luminosity where the H-burning shell contacts the chemical composition discontinuity produced when the convective envelope reaches its maximum penetration, is also provided for critical comparisons with observations.