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

Abstract

We report our study of features at the observed red end of the white dwarf\ncooling sequences for three Galactic globular clusters: NGC\\,6397, 47\\,Tucanae\nand M\\,4. We use deep colour-magnitude diagrams constructed from archival\nHubble Space Telescope (ACS) to systematically investigate the blue turn at\nfaint magnitudes and the age determinations for each cluster. We find that the\nage difference between NGC\\,6397 and 47\\,Tuc is 1.98$^{+0.44}_{-0.26}$\\,Gyr,\nconsistent with the picture that metal-rich halo clusters were formed later\nthan metal-poor halo clusters. We self-consistently include the effect of\nmetallicity on the progenitor age and the initial-to-final mass relation. In\ncontrast with previous investigations that invoked a single white dwarf mass\nfor each cluster, the data shows a spread of white dwarf masses that better\nreproduce the shape and location of the blue turn. This effect alone, however,\ndoes not completely reproduce the observational data - the blue turn retains\nsome mystery. In this context, we discuss several other potential problems in\nthe models. These include possible partial mixing of H and He in the atmosphere\nof white dwarf stars, the lack of a good physical description of the\ncollision-induced absorption process and uncertainties in the opacities at low\ntemperatures. The latter are already known to be significant in the description\nof the cool main sequence. Additionally, we find that the present day local\nmass function of NGC\\,6397 is consistent with a top-heavy type, while 47\\,Tuc\npresents a bottom-heavy profile.\n