Observations of ultraviolet carbon lines in the spectra of three DC white dwarfs.

Abstract

Ultraviolet spectra of three DC white dwarfs have been obtained using the International Ultraviolet Explorer satellite (IUE). These objects are LDS 275A (= EG 66 = 0935-37), G187-15 (=LTT16151 = EG262 = 2059+31), and L791-40 (=LTT9491 = EG264 = 2317-17). All three show the strong ultraviolet lines of neutral carbon recently found in the spectra of a number of other DC stars. Published observations of LDS 275B and G187-15 establish that they have continuous visual spectra, while recent scans of L791-40 and G187-15 made at Kitt Peak with the 2.1 m telescope and Image Intensified Dissector Scanner (lIDS) show weak λλ4472 and 5876 lines of neutral helium and possibly the (0, 0) Swan band of C₂, respectively. These new objects enlarge the sample of degenerates already known to have atmospheres containing carbon, either from visual or ultraviolet observations. When this type of white dwarf is plotted along with the other spectral classes in the Strömgren two-color diagram, it appears that white dwarfs observed having carbon occupy a restricted region, and no C₂ white dwarfs occur that are cooler than T_eff ≈ 6800 K. This is not consistent with the assumption that these white dwarfs are cooling at constant atmospheric carbon content and implies that a mechanism is present that decreases their relative atmospheric carbon abundances.