Isochrones and Luminosity Functions for Old White Dwarfs

Abstract

Using a new grid of models of cooling white dwarfs, we calculate isochrones and luminosity functions in the Johnson-Kron/Cousins and Hubble Space Telescope (HST) filter sets for systems containing old white dwarfs. These new models incorporate a nongray atmosphere that is necessary to properly describe the effects of molecular opacity at the cool temperatures of old white dwarfs. The various functions calculated and extensively tabulated and plotted are meant to be as utilitarian as possible for observers, so all results are listed in quantities that observers will obtain. The tables and plots developed should eventually prove critical in interpreting the results of HST's Advanced Camera observations of the oldest white dwarfs in nearby globular clusters, in understanding the results of searches for old white dwarfs in the Galactic halo, and in determining ages for star clusters using white dwarfs. As a practical application we demonstrate the use of these results by deriving the white dwarf cooling age of the old Galactic cluster M67.