A survey for cool white dwarfs and the age of the Galactic disc

We describe a new multicolour proper motion survey for cool white dwarfs (CWDs). The observational data base consists of ~ 300 digitally scanned Schmidt plates in ESO/SERC field 287. The entire survey procedure, from the raw Schmidt plate data to the final white dwarf luminosity function (WDLF), is described, with special emphasis on completeness concerns.

We obtain a sample of 58 WDs, for which we have follow-up CCD photometry and spectroscopy of a representative subsample. Effective temperatures and luminosities of our sample objects are determined by comparing photometry with the model atmosphere predictions of Bergeron, Saumon & Wesemael. Space densities are calculated using an adaptation of Schmidt's (1/V_max) method, from which a WDLF is constructed. Comparison of our observational LF with the models of both Wood and García-Berro et al. indicates an age for the local Galatic disc of 10^-1₊₃ Gyr. Importantly, we find no evidence of incompleteness in our survey sample. Proper motion number counts imply the survey is complete, and the WD sample passes the (V/V_max) completeness test.