Models for Old, Metal-Poor Stars with Enhanced alpha-Element Abundances. III. Isochrones and Isochrone Population Functions

Abstract

An improved algorithm for interpolating isochrones and isochrone population functions (IPFs), based on the scheme introduced by Bergbusch & VandenBerg (1992, APJS, 81, 163), is described. The computer programs (FORTRAN 77) and the grids of evolutionary tracks, for 17 [Fe/H] values between -2.31 and and -0.3, with three choices of [alpha/Fe] (0.0, 0.3, and 0.6) at each iron abundance, are freely available to users interested in producing isochrones and IPFs. We add to the evidence presented in previous papers in this series in support of the Teff and color scales of our models. The temperatures derived by Gratton et al. (1996, A&A, 314, 191) for local Population II subdwarfs with accurate {Hipparcos} parallaxes are in excellent agreement with those predicted for them, when the Gratton et al. [Fe/H] scale is also assumed. The locus defined by local subdwarfs and subgiants on the [M_V, log Teff] plane and the morphologies of globular cluster C-M diagrams are well matched by the present models, despite the neglect of diffusion. The three field halo subgiants in our sample all have ages >15 Gyr, which is also favored for the Galaxy's most metal-poor globular clusters (GCs). Our isochrones accurately reproduce the Da Costa & Armandroff (1990, AJ, 100, 162) red-giant branch fiducials for M15, NGC 6752, NGC 1851, and 47 Tuc on the [M_I, (V-I)_0]-diagram. However, our models fail to predict the observed luminosities of the red-giant bump by ~0.25 mag: this may indicate inward overshooting of convective envelopes in red giants. The Zinn & West (1984, ApJS, 55, 45) metallicities for intermediate metal-poor GCs (-1.8 > [Fe/H] > -1.1) are preferred over recent spectroscopic results, suggesting an inconsistency between current subdwarf and GC [Fe/H] scales.