Distance-independent absolute globular cluster ages from the color distribution of subgiant-branch stars

The distribution of subgiants with respect to color in the color-magnitude diagram of a globular cluster is found to be a reasonably sensitive function of age, with only a weak: dependence on metallicity. Such distributions are unaffected by interstellar reddening, provided that it is uniform across the face of the cluster, and they are completely independent of distance. The treatment of convection (e.g., the adopted value of the mixing length parameter) is a theoretical complication, but if this can be constrained by other observations, then ratios formed from the numbers of stars in different color bins along the subgiant branch can be used to determine absolute cluster ages. Following this approach, we derive an absolute age of 14 +/- 3 Gyr for M5 from a restricted sample of stars taken from the photometry of Sandquist et al. (1996, ApJ, 470, 910). We expect to reduce the uncertainty in such age estimates substantially with more populous samples of subgiant stars from this, and other clusters. (C) 1997 The American Astronomical Society. [S0004-6256(97)05012-7].