ar X iv : a st ro - p h / 98 02 21 7 v 1 1 6 Fe b 19 98 White dwarf stars and the Hubble Deep Field

The value of the Hubble Deep Field to study of the remote regions of the observable Universe is difficult to overstate, as the many exciting results of this workshop are ample testament. However, an added bonus of the HDF is that, although it a very narrow angle survey, the depth of the HDF results in its sampling a significant volume of the halo of our galaxy. Thus it is useful for the purposes of detecting (or placing upper limits on the distribution of) intrinsically faint stars, such as white dwarfs. In this capacity, the HDF can say some important things about the possible constituents of the baryonic dark matter in the halo of the Milky Way, as well as constrain the physics of white dwarf formation and cooling. Such an investigation is timely since white dwarfs could provide a significant fraction of the total mass of the halo of the Milky Way. The MACHO observations of microlensing events in the direction of the LMC suggests that up to 50% of the dark matter halo of the Milky Way could be comprised of faint Population II white dwarf stars (Alcock et al. 1997). Thus, constraints on the population of halo white dwarfs from the HDF can directly address this possible partial explanation of the nature of the dark halo of the Milky Way. In this review, I hope to illustrate how the HDF can be used to constrain the luminosity function of halo white dwarfs. I begin with a brief summary of the observed white dwarf luminosity function (WDLF) of the galactic disk, and show how the HDF serves as a probe of the WDLF for the halo. I then review the theoretical background used in interpreting the WDLF in terms of the theory of white dwarf evolution and cooling, and the history of star formation in the galaxy. We are then in a position to explore the theoretical WDLF, beginning with the WDLF of the disk. We then move to a theoretical examination of the WDLF of the halo population. The results of searches for white dwarfs on the HDF can then be examined in terms of the halo white dwarf population.