The space density of hot white dwarfs.

Preliminary results are given from one method of tackling the problem of setting constraints on the birth rate of main sequence progenitors. The space density of degenerate stars is important in galactic structure studies because white dwarfs represent a significant fraction of the local mass density and they act as tracers of the history of star formation. Combining the present day luminosity function with theoretical cooling tracks is necessary, but white dwarfs are intrinsically low luminosity objects so observational studies are hampered by difficulties in defining complete samples large enough to have statistical significance. Choosing a line of sight toward a nearby relatively dense dust cloud can reduce image crowding sufficiently to make possible surveys within the galactic plane. Applying the technique to stars that have the Coalsack Nebula in the background, five stars were found that were photometrically and spectroscopically good hot white dwarf candidates. If all five candidates are indeed degenerates, the inferred number of hot white dwarfs is 0.001 per cubic parsec. The sample is currently being extended to fainter apparent and absolute magnitudes as a more rigorous test of the validity and extent of the enhanced number density.