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 I and C II lines. These are together called DQ white dwarfs.
Aims: We want to derive atmospheric parameters T_eff, log g, 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 10 000 K have masses 0.4 M_⊙ larger than the classical DQ, which have masses typical for the majority of white dwarfs ( 0.6 M_⊙). 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 with T_eff > 14 500 K have atmospheres dominated by carbon.

Full Tables 1 and 2 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/628/A102