  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. Our analysis confirms that effective temperatures
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> = 0.62 Mo, is significantly lower than
that obtained from pure helium models, <M> = 0.73 Mo, and more consistent with
the spectroscopic mean mass of DB stars, <M> = 0.59 Mo, 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 Teff~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.
