Implications of the absolute magnitude distribution functions of DA and non-DA white dwarfs

The number distribution of absolute magnitudes in the sample of proper-motion-dominated white dwarfs of Liebert et al. (1983) and in the Palomar-Green sample is determined, and the distributions of DA and non-DA spectral types are analyzed on the basis of color-luminosity calibrations. The DA/non-DA ratios for the cool and hot ends of the distribution are found to be 2:1 and 6:1, respectively. Two mechanisms which can help explain the distribution functions are examined: a DA-to-DB transition at Te above 15,000 K via the diffusion-induced burning of surface H (as suggested by Michaud et al., 1984) and convective mixing of surface H in cooling DA stars at Te = 10,000-12,000 K.