Surface carbon abundances and compositional stratification of cool helium-rich white dwarfs.

Abstract

From the ultraviolet lines of CI in IUE spectra of the DQ white dwarfs L 845-70, GD 84, and G 188-27 we determine a surface carbon abundance of logC/He=-7, and T_eff_=9250, 9000 and 11000K. Comparison with the results for other stars of the same spectral class indicates variations in carbon abundance at the same temperature. If the carbon is the result of surface pollution by interior carbon dredged up by convection, we conclude that the variations cannot be caused by mass differences but probably by different outer helium layer thicknesses.