Models of Metal‐poor Stars with Gravitational Settling and Radiative Accelerations. II. The Age of the Oldest Stars

Isochrones for ages between 12 and 18 Gyr have been derived from the evolutionary tracks presented in Paper I (Richard et al.) for masses from 0.5 to 1.0 M☉ and initial chemical abundances corresponding to (1) Y = 0.2352, Z = 1.69 × 10-4 ([Fe/H] = -2.31, [α/Fe] = 0.3) and (2) Y = 0.2370, Z = 1.69 × 10-3 ([Fe/H = -1.31, [α/Fe] = 0.3). These are the first models for Population II stars in which both gravitational settling and radiative accelerations have been taken into account. Allowance for these diffusive processes leads to a 10%-12% reduction in age at a given turnoff luminosity. However, in order for the diffusive models to satisfy the constraints from Li and Fe abundance data (see Paper I) and to reproduce the observed morphologies of globular cluster (GC) color-magnitude diagrams (CMDs) in a straightforward way, extra mixing just below the boundary of the convective envelope seems to be necessary. Indeed, when additional turbulent mixing is invoked, the resultant models are able to satisfy all of these constraints, as well as those provided by the CMDs of local subdwarfs, rather well. Moreover, they imply an age near 13.5 Gyr for M92, which is one of the most metal-deficient (and presumably one of the oldest) of the Galaxy's GCs, if the field subgiant HD 140283 is used to derive the cluster distance. Comparisons of field subdwarfs and subgiants with a recently published fiducial for M5 suggests that the cluster has [Fe/H] ≲ -1.4, in conflict with some estimates based on high-resolution spectroscopy, if the metallicities of the field stars are to be trusted. In addition, an age of ≈11.5 Gyr is found for M5, irrespective of whether diffusive or nondiffusive isochrones are employed in the analysis. The implications of our results for the extragalactic distance scale and for the Hubble constant are briefly discussed in the context of the presently favored ΩM ≈ 0.35, ΩΛ ≈ 0.65 cosmological model.