The Luminosity Function of Hot DA White Dwarfs from an EUV-Selected Sample

The Extreme Ultraviolet Explorer (EUVE) all-sky survey has discovered a large number of hot DA white dwarfs (T_eff >= 25,000 K) that were not accounted for in previous determinations of the luminosity function and space density of these objects. The EUVE photometric data is sensitive to stellar effective temperatures but is also affected by biases, mainly interstellar medium (ISM) attenuation and photospheric impurities (Vennes & Fontaine 1992, ApJ, 401, 288). However, it is possible to model these effects and extract an homogeneous and relatively unbiased sample of objects. We derive a preliminary value of the space density of hot DA stars, in the range of effective temperatures T_eff >= 40,000 K, that appears to be roughly six times larger than the density obtained from the Palomar-Green survey by Fleming, Liebert, & Green (1986, ApJ, 308, 176). A remarkable finding is the virtual absence of DA white dwarfs hotter than ~ 65,000 K. The sampling is increasingly affected by ISM attenuation beyond an horizon of 80 pc. We have determined the following density for the range 40,000 <= T_eff <= 65,000 K: phi_ {>= 40,000 K} = 1.4 times 10(-5) pc(-3) ; and the total density in our sample is phi_ {>= 25,000 K} = 5.0 times 10(-5) pc(-3) . We obtained consistent results using the Wide Field Camera sample. Our new measurement of the space density of hot white dwarfs has implications regarding estimates of the white dwarf formation rate and the comparison with the planetary nebulae birthrate as well as for theoretical evolutionnary tracks. This work is supported by NASA contract NAS5-30180.