A critical examination of halo white dwarf candidates

A detailed analysis of halo white dwarf candidates is presented, which is based on model atmosphere fits to observed energy distributions built from photoelectric or photographic magnitudes. Most of the candidates identified in reduced proper motion diagrams are shown to be too warm (T-eff>5000 K) and most likely too young to be members of the Galactic halo, while the tangential velocities of the cooler and thus older white dwarfs are shown to be entirely consistent with the disk population. The results suggest that some white dwarf stars born in the young disk may have high velocities with respect to the local standard of rest. Such objects could represent the remnants of donor stars from close mass-transfer binaries that produced Type Ia supernovae via the single-degenerate channel or other scenarios suggested in the literature. Ongoing surveys that rely solely on reduced proper motion diagrams are likely to identify more of these high-velocity young degenerates, rather than to unveil the old white dwarf population of the Galactic halo. The importance of infrared photometry for studying extremely cool white dwarfs is also emphasized.