New DQ white dwarfs in the Sloan Digital Sky Survey DR4: confirmation of two sequences

Abstract

Using photometric selection criteria, and in a final step visual inspection of spectra, we find 65 DQ white dwarfs in the Data Release 4 of the Sloan Digital Sky Survey. 37 of these are new identifications, three more are reclassified DQ from DC, the other 25 have been described as DQ before in the literature. We derive effective temperatures and carbon abundances for 60 of these stars. The results confirm the main conclusions of Dufour et al.(2005). The majority of stars defines a clear sequence in the C abundance vs. Teff diagram, with high abundances found at high Teff and vice versa. We also confirm with high significance a second sequence with an abundance about 1 dex higher at the same Teff, and discuss the nature and possible origin of the high-C sequence.