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NGC 2168:LAWDS 28

Title: A Hot DQ White Dwarf in the Open Star Cluster M35
Authors: Kurtis A. Williams (1), James Liebert (1), Michael Bolte (2), Robert B. Hanson (2) ((1) Steward Observatory, (2) UCO/Lick Observatory)

Researchers report the discovery of a hot DQ white dwarf, NGC 2168:LAWDS 28, that is a likely member of the 150-million year old cluster NGC 2168 (Messier 35). The spectrum of the white dwarf is dominated by CII features. The effective temperature is difficult to estimate but likely > 20,000 K based on the temperatures of hot DQs with similar spectra.
NGC2168:LAWDS 28 provides further evidence that hot DQs may be the "missing" high-mass helium-atmosphere white dwarfs.
Based on published studies, they 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 white dwarfs in open clusters with turnoff masses > 2 solar masses. Based on the number of known cluster DA white dwarfs and a redetermination of the H-atmosphere:He-atmosphere ratio, commonly known as the DA:DB ratio, the researchers re-examine the hypothesis that the H- to He-atmosphere ratio in open clusters is the same as the ratio in the field. Under this hypothesis, they 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 ~ 2-sigma level.

Spectrum of NGC 2168:LAWDS 28 (top) compared to the known hot DQ SDSS
J010647.9+151327.9 (bottom).
Dashed tick marks indicate the location of C II features. The qualitative match of the two spectra and the fact that most or all of the features in the top spectrum match the C II features lead the researchers to conclude that NGC2168:LAWDS 28 is a hot DQ white dwarf.

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