A LOWER LIMIT OF 9.5 GYR ON THE AGE OF THE GALACTIC DISK FROM THE OLDEST WHITE-DWARF STARS

WHITE dwarf stars represent the final evolutionary state for most main -sequence stars. They cool slowly enough that even the oldest white dw arfs are still observable in sufficiently deep surveys and they theref ore provide a record of the age and star-formation history of the Loca l disk of the Milky Way(1-7)-and hence a useful constraint on the age of the Galaxy itself. Here we report the initial results of a very dee p survey of white dwarfs, that avoids many of the problems associated with the incompleteness of earlier surveys. We use model age-luminosit y relations to interpret the luminosity function of our sample of star s, and thus obtain a minimum age for the local Galactic disk of simila r to 9.5 Gyr, Our results lend weight to an emerging picture of the ev olutionary history of the Milky Way, in which the halo formed similar to 14-17 Gyr ago(8,9), followed by the bulge globular dusters similar to 12-14 Gyr ago(10), with a modest hiatus before the onset of star fo rmation in the local disk similar to 10 Gyr ago.