The Accreting White Dwarf in the Close Binary EUVE J1016−053

Abstract

The hot white dwarf in the close binary EUVE J1016-053 (=RE 1016-053) has been classified as a DAO white dwarf; its mixed H/He composition has been attributed to steady accretion from the close red dwarf companion. We have obtained extreme-ultraviolet (EUV) photometric and spectroscopic observations of EUVE J1016-053 with the Extreme Ultraviolet Explorer (EUVE). We report the discovery of a ≈ 30% EUV flux variation over a period of 57.3 minutes, which we attribute to surface abundance inhomogeneities modulated over the stellar rotation period, Prot. The EUVE spectrum shows the effect of heavy-element opacities on the white dwarf EUV energy distribution. Spectral synthesis including trace opacities of helium and a group of heavy elements (C, N, O, Si, S, Fe) in the otherwise hydrogen-rich atmosphere constrains abundances to Y/Y☉ = Z/Z☉ = 2 × 10-3, in support of a simple accretion model. The low surface-averaged helium abundance measured in the white dwarf atmosphere limits the accretion rate to between 10-19 and 10-18 M☉ yr-1, i.e., much lower than the Bondi-Hoyle accretion rate, which is of the same order as the red dwarf mass-loss rate (≥10-14 M☉ yr-1), therefore invalidating a simple wind-accretion model. We speculate that weak mass loss from the white dwarf or interaction with a magnetosphere may inhibit accretion onto the white dwarf. Accretion of heavy elements may also be restricted to smaller areas, possibly correlating to magnetic poles.