THICK TO THIN - THE EVOLUTIONARY CONNECTION BETWEEN PG-1159 STARS AND THE THIN HELIUM-ENVELOPED PULSATING WHITE-DWARF GD-358

Seismological observations with the Whole Earth Telescope (WET) allow the determination of the subsurface compositional structure of white dwarf stars, The hot DO PG 1159-035 has a helium surface layer with a mass of similar to 10(-3) M(.), while the cooler DB white dwarf GD 358 has a much thinner surface helium layer of 10(-6) M(.). Taken literally, these results imply that either there is no evolutionary relation between these two stars, or that there is an unknown mass-loss mechanism. In order to investigate a possible evolutionary link between these objects, we computed evolutionary sequences of white dwarf models that included time-dependent diffusive processes. We used an initial model based on the PG 1159 pulsational data, which has a surface layer 3 x 10(-3) M(.) thick, and a composition of 30% helium, 35% carbon, and 35% oxygen. Below this surface layer is a thin transition zone where the helium fraction falls to zero. As expected, diffusion caused a separation of the elements; a thickening surface layer of nearly pure helium overlays a deepening transition zone where the composition changes to the surface composition of the original model. When the model reached the temperature range inhabited by GD 358 and the pulsating DB white dwarfs, this pure helium surface layer was similar to 10(-5.5) M(*) deep. The resulting evolved model is very similar to the model used by Bradley and Winget to match the pulsation observations of GD 358. The pulsation periods of this model also show a good fit to the WET observations. These results demonstrate the plausibility of a direct evolutionary path from PG 1159 stars to the much cooler DB white dwarfs by inclusion of time dependent diffusion. A problem still remains in that our models have no hydrogen, and thus must retain their DB nature while evolving through the T-eff 45,000 to 30,000 K. Since there are no known DB stars in this range, we plan to address this problem in future calculations.