2005ApJ...627..404D

We present an analysis of spectroscopic and photometric observations of cool DQ white dwarfs based on improved model atmosphere calculations. In particular, we revise the atmospheric parameters of the trigonometric parallax sample of Bergeron, Leggett, & Ruiz and discuss the astrophysical implications on the temperature scale and mean mass, as well as the chemical evolution of these stars. We also analyze 40 new DQ stars discovered in the First Data Release of the Sloan Digital Sky Survey (SDSS). Our analysis confirms that effective temperatures (T_eff) derived from model atmospheres including carbon are significantly lower than the temperatures obtained from pure helium models. Similarly, the mean mass of the trigonometric parallax sample, <M≥0.62M_☉, is significantly lower than that obtained from pure helium models, <M≥0.73M_☉, and more consistent with the spectroscopic mean mass of DB stars, <M≥0.59M_☉, the most likely progenitors of DQ white dwarfs. We find that DQ stars form a remarkably well-defined sequence in a carbon abundance versus effective temperature diagram; below T_eff∼10,000 K, carbon pollution decreases monotonically with decreasing effective temperature. Improved evolutionary models including diffusion and connecting to the PG 1159 phase are used to infer a typical value for the thickness of the helium layer M_He/M_*between 10^–3 and 10^–2, compatible with the predictions of post-AGB models. Several DQ stars in our sample, however, show larger than average carbon abundances. We argue that these DQ stars are all massive white dwarfs and could represent the high-mass tail of the white dwarf mass distribution, with their hotter counterparts corresponding to the hot DQ stars reported recently by Liebert et al. The number distribution of DQ white dwarfs as a function of effective temperature clearly shows a sudden drop at about T_eff∼7000 K and an abrupt cutoff at T_eff∼6000 K. The existence of this cutoff is now statistically more significant with the addition of the SDSS stars. The physical mechanism responsible for this cutoff is still unknown, even though it is believed to be somehow related to the existence of the so-called C₂H stars at lower temperatures.