Detection of Photospheric O VI in the Fuse Spectra of Low Metallicity White Dwarfs

Abstract

The Far Ultraviolet Spectroscopic Explorer (FUSE) has opened a new spectral window that permits the study of important ions such as OVI, PV, and SVI in FUV spectra of hot white dwarfs. The detection of the OVI 1031.912 Å and 1037.613 Å absorption lines in the FUSE spectra of Lan 23, GD 984, RE J1032+532, and RE J2156-543 came as a surprise, as these stars have effective temperatures ranging from 45,000 K to 60,000 K, which are on the cold side regarding the detection of the high ionization OVI ion. Even more puzzling, given the strength of the OVI lines, no OIV and OV lines are detected in the STIS high resolution spectra of RE J1032+532 and RE J2156-543, and no oxygen photoionization edges are observed in the EUVE data of GD 984, RE J1032+532 and RE J2156-543. In a combined analysis of FUSE, STIS, and EUVE data, we came to the conclusion that the oxygen abundance in the atmospheres of these stars is stratified.