A hot DQ white dwarf in the open star cluster M35

We report the discovery of a hot DQ white dwarf, NGC 2168: LAWDS 28, that is a likely member of the 150 Myr old cluster NGC 2168 (M35). The spectrum of the white dwarf (WD) is dominated by C II features. The effective temperature is difficult to estimate but likely greater than or similar to 20,000 K based on the temperatures of hot DQ WDs with similar spectra. NGC 2168: LAWDS 28 provides further evidence that hot DQ WDs may be the "missing" high-mass helium-atmosphere white dwarfs. Based on published studies, we find that the DBA WD LP 475-242 is likely a member of the Hyades open cluster, as often assumed. These two white dwarfs are the entire sample of known He-atmosphere WDs in open clusters with turnoff masses >= 2 M(.). Based on the number of known cluster DA white dwarfs and a redetermination of the H-atmosphere to He-atmosphere ratio, commonly known as the DA: DB ratio, we reexamine the hypothesis that the H- to He-atmosphere ratio in open clusters is the same as the ratio in the field. Under this hypothesis, we calculate that five He-atmosphere WDs are expected to have been discovered, with a probability of finding fewer than three He-atmosphere white dwarfs of 0.08, or at the approximate to 2 sigma level.