M4 - A Globular Cluster Hubble Deep Field: The White Dwarf Sequence

We report on our analysis of the white dwarf cooling sequence detected to V 30 in our 123 orbit HST pointing at a field in the globular cluster M4 (GO-8679). Cluster membership of many white dwarfs is confirmed by their common proper motion, using earlier HST data. We present the white dwarf cooling sequence as well as a detailed comparison with atmospheric and evolutionary models which enables us to place constraints on the age of M4 as well as the main sequence - white dwarf mass relation. We will also discuss the influence of hydrogen and helium atmospheres on the cooling and appearance of the white dwarfs and will constrast the M4 white dwarfs with those in the Galactic Disk as well as the recently reported population of high velocity field white dwarfs. In addition to its importance as a cosmochronological testbed, the M4 white dwarf sequence thus provides valuable insight into the search for dark matter and/or the microlensing population.