Atmospheric parameters and carbon abundance of white dwarfs of spectral types C2 and DC.

Abstract

Model atmosphere calculations over a wide range of effective temperatures and carbon abundances are used to analyze IUE data obtained between 1200 and 3200 A as well as photometric and spectroscopic data in the visible region for several sets of white dwarfs of spectral types C2 and DC. The carbon abundances relative to helium are found to range from 0.0000001 to 0.000001 for the DC stars and from 0.000001 to 0.002 for the C2 stars. Upper limits on the abundances of O, N, H, Mg, and Ca are discussed, and the origin of the observed carbon abundances is considered. Masses are derived for 17 DC and C2 stars with known distances. A mean value of 0.55 solar mass is obtained, which is comparable to the masses reported for DA white dwarfs.