OXYGEN-ENHANCED MODELS FOR GLOBULAR-CLUSTER STARS .2. ISOCHRONES AND LUMINOSITY FUNCTIONS

Isochrones and luminosity functions (LFs) on the BY photometric system have been constructed forages from 10 to 18 Gyr for each of the seven values of [Fe/H] = -2.26, -2.03, -1.78, -1.66, -1.48, -1.26,and -1.03,and for ages from 5 to 18 Gyr for [Fe/H] = -0.78, -0.65, and -0.47. The loci forages of 10 Gyr or more have been provided mainly for application to photometry of the Galactic globular clusters (GCs); the rest should be useful for interpreting data for old, metal-poor open clusters. Oxygen-to-iron ratios have been assumed that closely follow the relation [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. The helium abundance was assumed to vary from Y = 0.2350 to 0.2454 over the adopted range in [Fe/H] while scaled-solar abundances were assumed for all other elements. A new interpolation code was used to generate the isochrones and LFs: as fully described herein, it is based on the technique first developed by Prather, but by using (among other refinements) an improved procedure to select the so-called "Equivalent Evolutionary Phase" points, nothing more sophisticated than linear interpolation is needed to produce exceedingly smooth and accurate results. Comparisons of the isochrones are made with the observed fiducial sequences for a few GCs (specifically M92, M13, and 47 Tuc) and with a number of the Population II subdwarf standards that have accurate parallax-based distances, to demonstrate that they are consistent with these data within current uncertainties. Illustrative fits of the computed luminosity functions, which are provided for several power-law mass functions, to the observed LFs recently reported in the literature for M92, M13, and 47 Tuc are also presented and discussed. The calculations are extensively tabulated: they should be particularly useful for deter-mining the ages of the GCs, and for population synthesis analyses, given that they are predicted for the entire magnitude range -3 less than or similar to M(V) less than or similar to 12.