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 v 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.