CONSTRAINTS ON THE AGE AND EVOLUTION OF THE GALAXY FROM THE WHITE-DWARF LUMINOSITY FUNCTION

The white dwarf stars are the cooling electron-degenerate stellar remnants of some 98% of all mainsequence stars formed, and they cool slowly enough that even those that are remnants of the earliest generation of stars in the local Galactic disk have not had time to cool to invisibility. As a result of this, the white dwarf luminosity function has written in it the age and star formation history of the Galaxy at our galactocentric radius. Using the best available inputs, 1 present an extensive exploration of the white dwarf disk luminosity function using the observational results of Liebert, Dahn, & Monet as a template for comparison, and the cooling curves of Wood as the input basis functions for the integration. The results are particularly sensitive to the star formation rate as a function of time, the internal core composition, and the mass of the surface helium layer. They are relatively insensitive to uncertainties in the initial mass function, the white dwarf mass function within current error estimates, and the upper limit to the masses of the main-sequence progenitors. The inclusion of plausible scale-height inflation for the oldest objects changes somewhat the shape of the luminosity functions at the turndown but does not drastically alter the derived age estimates. The results suggest that the star formation rate over the history of the Galaxy has been constant to within an order of magnitude, and that conservatively, the disk age lies in the range 6-13.5 Gyr, where roughly 40% of the uncertainly is due to the observational uncertainties. The principal uncertainties are the core composition and surface helium layer mass-which scismological observations will severely constrain in the coming years-and the bolometric corrections appropriate to the cool white dwarfs. Using the best current estimates as inputs to the integration, the disk ages range from 7.5 to 11 Gyr; i.e., they are substantially younger than most estimates for the halo globular clusters but in reasonable agreement with those for the disk globular clusters and open clusters. The ages of these differing populations, taken together, are consistent with the pressure-supported collapse models of early spiral galactic evolution.