A Critical Examination of Halo White Dwarf Candidates

A detailed analysis of halo white dwarf candidates is presented, which isbased on model atmosphere fits to observed energy distributions built fromphotoelectric or photographic magnitudes. Most of the candidates identified inreduced proper motion diagrams are shown to be too warm (Teff > 5000 K) andmost likely too young to be members of the galactic halo, while the tangentialvelocities of the cooler and thus older white dwarfs are shown to be entirelyconsistent with the disk population. The results suggest that some white dwarfstars born in the young disk may have high velocities with respect to the localstandard of rest. Such objects could represent the remnants of donor stars fromclose mass-transfer binaries that produced type Ia supernovae via the singledegenerate channel, or other scenarios suggested in the literature. Ongoingsurveys that rely solely on reduced proper motion diagrams are likely toidentify more of these high velocity young degenerates, rather than to unveilthe old white dwarf population of the galactic halo. The importance of infraredphotometry for studying extremely cool white dwarfs is also emphasized.Comment: Accepted for publication in the Astrophysical Journal, 16 pages, 9 figure