DISCOVERY OF PHOTOSPHERIC SULFUR AND PHOSPHORUS IN ORFEUS SPECTRA OF THE HOT WHITE-DWARFS G191-B2B AND MCT-0455-2812

High-dispersion observations of the far-ultraviolet (FUV) spectra of t he hot DA white dwarfs G191-B2B and MCT 0455-2812 reveal the presence of two previously undetected elements in white dwarf photospheres: sul fur and phosphorus. The spectra, obtained with the Berkeley EUV/FUV sp ectrometer and the ORFEUS telescope aboard the space platform Astro-SP AS, show the characteristic Lyman series and extend beyond the H I int erstellar medium (ISM) absorption edge in the extreme ultraviolet. We provide new effective temperature and surface gravity measurements bas ed on a detailed fit to the Lyman series that show an interesting depe ndence on the heavy-element abundance. Weak absorption lines are ident ified with C III lambda 977.02, N III lambda 989.799, Si IV lambda lam bda 1066.629, 1122.486, 1128.325, P V lambda lambda 1117.978, 1128.007 , and S IV lambda lambda 1062.671, 1072.990. The origin of the carbon and nitrogen features is uncertain, but the silicon, phosphorus, and s ulfur detection provides the basis for new photospheric abundance meas urements: 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 als o 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 deple tion possibly associated with previous mass-loss episodes in these hot , therefore young, white dwarfs. On the other hand, we confirm an exce ss of silicon that could be explained if these stars currently accrete solid particles from their immediate environment.