ASTEROSEISMOLOGY OF WHITE-DWARF STARS .1. ADIABATIC RESULTS

We report the results of our preliminary investigation of the asteroseismological properties of chemically stratified evolutionary DA and DB white dwarf models. We computed the period and kinetic energy distributions for nonradial g-modes of spherical harmonic indices l = 1-3 in the adiabatic approximation. Using an extensive grid of models, we studied the effects of varying the total stellar mass and the surface layer masses on their pulsation properties. We find significant resonant mode trapping cause by the chemical stratification of our models. Modes trapped in the outer layers have much lower kinetic energies; these kinetic energy minima also show up as minima in the period spacing between modes of consecutive radial overtone, k. Mode trapping occurs at the same or nearly the same value of k for different l-values. This means that the l-values of trapped modes may be identified on the basis of their period ratios. We demonstrate that we can use observational identification of these period ratios to constrain the mass of the star and its surface layers. These results suggest that we can use the adiabatic pulsation properties of white dwarf models to determine the interior structure of the DAV and DBV stars. Finally, we exploit the evolutionary nature of our models to compute rates of period change (II) to compare with observations. For our carbon-core DAV models, these values are a factor of 5-10 lower than the best current upper limits. Our theoretical values probably represent lower limits for the expected values because of our choice of carbon cores and neglect of neutrino energy losses (important only in the DBV models).