SIMBAD references

Context. Among the spectroscopically identified white dwarfs, a fraction smaller than 2% have spectra dominated by carbon lines, mainly molecular C₂, but also a smaller group dominated by CI and CII lines. These are together called DQ white dwarfs.
Aims. We want to derive atmospheric parameters T_eff, logg, and carbon abundances for a large sample of these stars and discuss implications for their spectral evolution.
Methods. Sloan Digital Sky Survey spectra and ugriz photometry were used, together with Gaia Data Release 2 parallaxes and G band photometry. These were fitted to synthetic spectra and theoretical photometry derived from model atmospheres. Results. We found that the DQ hotter than T_eff∼10000K have masses ∼0.4M_☉ larger than the classical DQ, which have masses typical for the majority of white dwarfs (∼0.6M_☉). We found some evidence that the peculiar DQ below 10000K also have significantly larger masses and may thus be the descendants of the hot and warm DQ above 10000K. A significant fraction of the hotter objects with T_eff>14500K have atmospheres dominated by carbon.