A comparative analysis of the observed white dwarf cooling sequence from globular clusters

We report our study of features at the observed red end of the white dwarf cooling sequences for three Galactic globular clusters: NGC 6397, 47 Tucanae and M 4. We use deep colour-magnitude diagrams constructed from archival Hubble Space Telescope (Advanced Camera for Surveys) to systematically investigate the blue turn at faint magnitudes and the age determinations for each cluster. We find that the age difference between NGC 6397 and 47 Tuc is 1.98(-0.26)(+0.44) Gyr, consistent with the picture that metal-rich halo clusters were formed later than metal-poor halo clusters. We self-consistently include the effect of metallicity on the progenitor age and the initial-to-final mass relation. In contrast with previous investigations that invoked a single white dwarf mass for each cluster, the data show a spread of white dwarf masses that better reproduce the shape and location of the blue turn. This effect alone, however, does not completely reproduce the observational data - the blue turn retains some mystery. In this context, we discuss several other potential problems in the models. These include possible partial mixing of H and He in the atmosphere of white dwarf stars, the lack of a good physical description of the collision-induced absorption process and uncertainties in the opacities at low temperatures. The latter are already known to be significant in the description of the cool main sequence. Additionally, we find that the present-day local mass function of NGC 6397 is consistent with a top-heavy type, while 47 Tuc presents a bottom-heavy profile.