The frequency of hydrogen white dwarfs as observed at high signal to noise ratio

Despite their relatively high stability and slow evolution, white dwarfs are subject to important changes as they cool, since little material is required to contaminate an atmosphere that is only a few meters thick. With an eye to these complexities, about 140 white dwarfs were observed on eight nights in 1982, with about 10,000 to 30,000 photons/pixel being detected near the central wavelengths of the double CCD camera used. Steep Balmer decrements are noted to occur in the cooler degeneration due to increasing He/H ratio. The predominance of non-DA stars as late evolutionary stages prompts questions as to the fate of the accreted and residual H in postasymptotic giant branch evolution. No definitive theoretical argument is found to explain how most of the residual hydrogen is destroyed.