Hot White Dwarfs in the Extreme ‐ Ultraviolet Explorer Survey. IV. DA White Dwarfs with Bright Companions

We present an analysis of optical, ultraviolet, and X-ray spectral properties of a sample of 13 hot hydrogen-rich (DA) white dwarfs, each paired with a luminous unresolved companion. Using low-dispersion International Ultraviolet Explorer spectra, ROSAT photometry, and Extreme-Ultraviolet Explorer photometry and spectroscopy, we estimate the effective temperature, mass, and distance of the white dwarfs. Additionally, we examine the question of their atmospheric composition. We establish orbital properties for most binaries by means of high-dispersion optical spectroscopy obtained with the Hamilton echelle spectrograph at Lick Observatory; the same data help uncover evidence of activity in some of the secondary stars that is also notable in ROSAT X-ray measurements. In particular, we find high-amplitude (\u3e20 km s-1) velocity variations in only two stars (HD 33959C and HR 8210), low-amplitude variations in four additional objects (HD 18131, HR 1608, θ Hya, and BD +27°1888), and no variations (\u3c2 km s-1) in the remainder. We have observed Ca H and K in emission in four (BD +08°102, HD 18131, HR 1608, and EUVE J0702+129) of the six objects that were also detected in the 0.52-2.01 keV ROSAT PSPC band, while the source of the hard X-ray emission in HD 33959C remains unknown; other investigators have noted some evidence of activity in the remaining 0.52-2.01 keV detection, HD 217411. Properties of the white dwarfs are also investigated; EUV spectroscopy shows the effect of a low heavy element abundance in the atmosphere of the white dwarf in HD 33959C and of a high heavy element abundance in HD 223816; measurements of all other objects are apparently consistent with emission from pure-hydrogen atmospheres. However, current data do not constrain well the white dwarf parameters, and, to remedy the situation, we propose to obtain spectroscopy of the complete H Lyman line series