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

¹ Institut für Theoretische Physik und Astrophysik, Universität Kiel, 24098 Kiel, Germany
e-mail: koester@astrophysik.uni-kiel.de
² Instituto de Fisica, Universidade Federal do Rio Grande do Sul, 91501-900 Porto-Alegre, RS, Brazil

Received: 24 May 2019
Accepted: 26 June 2019

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.