The Cool White Dwarf Luminosity Function and the Age of the Galactic Disk

Abstract

We present new optical and infrared data for the cool white dwarfs in the proper motion sample of Liebert, Dahn, & Monet. Stellar properties—surface chemical composition, effective temperature, radius, surface gravity, mass, and luminosity—are determined from these data by using the model atmospheres of Bergeron, Saumon, & Wesemael. The space density contribution is calculated for each star and the luminosity function (LF) for cool white dwarfs is determined. Comparing the LF to the most recent cooling sequences by Wood implies that the age of the local region of the Galactic disk is 8 ± 1.5 Gyr. This result is consistent with the younger ages now being derived for the globular clusters and the universe itself.