RE-1016-053 - A PRE-CATACLYSMIC BINARY, AND THE 1ST EXTREME ULTRAVIOLET AND X-RAY DETECTIONS OF A DAO WHITE-DWARF

Multiwavelength observations have been obtained of the EUV/x-ray sourc e RE 1016-053. Optical spectra exhibit the steep blue continuum and Ba lmer absorption typical of a hot white dwarf, but there are bright, na rrow emission lines of H I, He I, and Ca II superimposed. These lines were observed to undergo a dramatic change in strength. Such behavior is characteristic of precataclysmic binaries (PCBs) like Feige 24, cau sed by substantial heating of the near side of a close red dwarf compa nion. However, in the case of RE 1016-053 the continuum of the seconda ry is undetected. A clear He II 4686 angstrom absorption line indicate s that the white dwarf is a hydrogen-helium (DAO) hybrid star, and is the first to have been detected at EUV/x-ray wavelengths. Including RE 1016-053, four of the five PCBs with white dwarfs hotter than 40 000 K appear to be DAOs, leading to the suggestion that mass loss on the A symptotic Giant Branch (AGB) may have been significantly different in these systems compared to isolated DA stars. The temperature of RE 101 6-053 is found to be 55 800 +/- 1000 K, and the surface gravity log g = 7.81 +/- 0.07, from which a mass M = 0.57 +/- 0.03 M. has been deriv ed. There is also an unusually strong C IV absorption feature at 1550 angstrom, which may be predominantly circumstellar. RE 1016-053 is by far the brightest of this class of objects in the 60-140 angstrom rang e, but like most other H-rich white dwarfs hotter than 40 000 K, corre ctly modeling the EUV/x-ray data requires photospheric opacity sources in addition to H and He, probably heavy elements.