Discovery of Photospheric Sulfur and Phosphorus in ORFEUS Spectra of the Hot White Dwarfs G191-B2B and MCT O455-2812
High-dispersion observations of the far-ultraviolet (FUV) spectra of the hot DA white dwarfs G191-B2B and MCT 0455-2812 reveal the presence of two previously undetected elements in white dwarf photospheres: sulfur and phosphorus. The spectra, obtained with the Berkeley EUV/FUV spectrometer and the ORFEUS telescope aboard the space platform Astro-SPAS, show the characteristic Lyman series and extend beyond the H I interstellar medium (ISM) absorption edge in the extreme ultraviolet. We provide new effective temperature and surface gravity measurements based on a detailed fit to the Lyman series that show an interesting dependence on the heavy-element abundance. Weak absorption lines are identified with C III λ977.O2, N III λ989.799, Si IV λλ1O66.629, 1122.486, 1128.325, P V λλ1117.978, 1128.007, and S IV λλ1O62.671, 1072.990. The origin of the carbon and nitrogen features is uncertain, but the silicon, phosphorus, and sulfur detection provides the basis for new photospheric abundance measurements: log (Si/H) = -6.4, log (P/H) = -7.7, and log (S/H) = -6.7 in G191-B2B, and log (Si/H) = -5.7 and log (P/H) = -7.6 in MCT 0455-2812. We also provide upper limits to the abundance of helium and chlorine. These measurements provide a critical insight into the spectral evolution of white dwarf stars, and we find strong evidence of heavy-element depletion possibly associated with previous mass-loss episodes in these hot, therefore young, white dwarfs. On the other hand, we confirm an excess of silicon that could be explained if these stars currently accrete solid particles from their immediate environment.