Hot white dwarfs as soft X-ray sources. II - The space density of hot white dwarfs determined from soft X-ray surveys

New limits on the space density of hot white dwarfs are derived from a soft X-ray survey and from recent theoretical work on hot, high-gravity atmospheres. The observed count yields a value that is less than approximately 3.3 x 10 to the -7/cu pc at an effective temperature of 70,000 K for an assumed white dwarf mass equal to 0.6 solar mass and a uniform average interstellar neutral hydrogen density of 0.15/cu cm. It is noted that within the uncertainties in both the theoretical expectations and the observed upper limits, this result is compatible with the predictions of white dwarf cooling theory including neutrino processes. The observed space density at an effective temperature greater than or equal to 70,000 K, however, cannot be used to exclude evolution without neutrino energy losses.