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Abstract

Several model atmosphere grids are produced for effective temperatures, surface gravities, arid metallic and hydrogen abundances covering the parameter space where these stars are found. We develop the necessary tools (minimization techniques) for an accurate and systematic determination of the best solution for each star in our sample. This sample consists of the stars from the studies of Bergeron et al. (1997, 2001) as well as the recent discoveries from the Sloan Digital Sky Survey (SDSS). Additional optical spectroscopic observations were also obtained from runs at the Observatoire du niont Megantic and the Steward Observatory at Kitt Peak.

We find atmospheric parameters for 56 DQ stars and 159 DZ stars; the largest samples ever analyzed in a homogeneous fashion. Good fits are obtained from our models except for the coolest objects which are affected by high-density effects. Masses are obtained for 17 DQ and 11 DZ stars with trigonometric parallax measurements. We find that the mean masses of these stars are comparable to those of DA and DB white dwarfs obtained in several previous studies.

Moreover, we find that the thickness of the helium layer for the bulk of our DQ sample is more compatible with post-AGB predictions. Additionally, we reveal the existence of a second sequence of DQ stars with a higher carbon abundance by about 1 dex. We also present the first detailed analysis of a magnetic DZ white dwarf.

Finally, our analysis of DZ white dwarfs reveals a correlation between the effective temperature and the hydrogen abundance. This correlation suggests that perhaps these stars are the result of the convective mixing of a DA white dwarf atmosphere with its underlying helium layer.