2006A&A...454..951K

DQ white dwarfs show atomic or molecular carbon features in their spectra. The atmosphere consists of helium and the carbon is believed to be dredged-up to the surface by the deepening helium convection zone. We want to identify new DQ in the Data Release 4 of the Sloan Digital Sky Survey, determine the effective temperatures and carbon abundances and search for systematic trends between these parameters as indication of the origin of this class. Photometric selection criteria are developed and applied to the catalog to identify candidates, which are in a final step confirmed by visual inspection of spectra. The parameters are determined through comparison with theoretical spectra and colors. Our final DQ catalog contains 65 stars. 40 of these are new identifications, 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 majority of stars defines a clear sequence in the C abundance vs. T_eff diagram, with high abundances found at high T_eff and vice versa. We also confirm with high significance a second sequence with an abundance about 1dex higher at the same T_eff, and discuss the nature and possible origin of the high-C sequence.