Carbon-rich (DQ) white dwarfs in the Sloan Digital Sky Survey

Abstract

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 C Iand C IIlines. These are together called DQ white dwarfs.Aims.We want to derive atmospheric parametersTeff, logg, and carbon abundances for a large sample of these stars and discuss implications for their spectral evolution.Methods.Sloan Digital Sky Survey spectra andugrizphotometry were used, together withGaiaData Release 2 parallaxes andGband photometry. These were fitted to synthetic spectra and theoretical photometry derived from model atmospheres.Results.We found that the DQ hotter thanTeff~ 10 000 K 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 10 000 K also have significantly larger masses and may thus be the descendants of the hot and warm DQ above 10 000 K. A significant fraction of the hotter objects withTeff> 14 500 K have atmospheres dominated by carbon.