Properties of hot white dwarfs in extreme-ultraviolet/soft X-ray surveys

Intermediate-dispersion spectroscopy (Δ ∼ 6 Å) of 38 ultrasoft X-ray sources (ROSAT PSPC/WFC and EUVE) - 27 H-rich white dwarfs (DA), one magnetic white dwarf (DAp), eight active galactic nuclei, a new cataclysmic variable, and an active late-type star - is presented. Atmospheric (T_eff, log g) and stellar (age, mass) parameters of the DA white dwarfs are determined, for the first time in the case of 12 objects. Adding the present sample to the EUV-selected sample of previous studies by Vennes et al., I define an enlarged population of 141 hot white dwarfs and redetermine the DA white dwarf mass distribution taking into account improved mass measurements for ultramassive white dwarfs. High-dispersion spectroscopy (Δ ∼ 1.0-1.4 Å) of the Hα line core in a representative collection of white dwarfs (the ultramassive DA GD 50, two hot DAs, and the low-mass DA EUVE J0512-006) indicate low projected rotation velocity υ_rot sin i ≤ 30-65 km s^-1 and no perceptible radial velocity variations in the low-mass white dwarf EUVE J0512-006. Questions on the origin and evolution of hot white dwarf stars are examined in light of these results.