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 alikely member of the 150-Myr old cluster NGC 2168 (Messier 35). The spectrum ofthe white dwarf is dominated by CII features. The effective temperature isdifficult to estimate but likely > 20,000 K based on the temperatures of hotDQs with similar spectra. NGC2168:LAWDS 28 provides further evidence that hotDQs may be the ``missing'' high-mass helium-atmosphere white dwarfs. Based onpublished studies, we find that the DBA WD LP 475-242 is likely a member of theHyades open cluster, as often assumed. These two white dwarfs are the entiresample of known He-atmosphere white dwarfs in open clusters with turnoff masses>2 solar masses. Based on the number of known cluster DA white dwarfs and aredetermination of the H-atmosphere:He-atmosphere ratio, commonly known as theDA:DB ratio, we re-examine the hypothesis that the H- to He-atmosphere ratio inopen clusters is the same as the ratio in the field. Under this hypothesis, wecalculate that five He-atmosphere WDs are expected to have been discovered,with a probability of finding fewer than three He-atmosphere white dwarfs of0.08, or at the ~ 2-sigma level.Comment: 9 pages with 2 figures, uses LaTeX/AASTeX 5.2. Accepted for publications in ApJ Letter