Asteroseismology of the DB Star GD 358: Improved Evolutionary Models

Abstract

We have carried out detailed evolutionary calculations of white dwarf models which naturally explain the PG1159-DB-DQ connection. Our evolutionary code (briefly described in Fontaine, Brassard, & Bergeron 2001) was used with the diffusion option turned on. We started our sequences with very hot models having an uniform envelope composition made of a mixture of He, C, and O, similar to the compositions observed in the atmospheres of real PG1159 stars. With passing time, C and O in the envelope settle down and leave behind a pure He mantle which thickens with cooling. Contrary to what has been very unfortunately propagated in the literature since 1994, He separation is still far from over by the time a model has cooled to an effective temperature (∼ 25,000 K) characteristic of the pulsating DB white dwarfs, and, furthermore, the He mantle continues to thicken even down to the DQ range (∼ 10,000 K and cooler). Thus, the “conflict” referred to in a good dozen papers between the “thin” He layers in pulsating DB’s -as inferred from asteroseismology- and the “thick” He layers in DQ’s -as inferred from the observed abundances of carbon in DQ stars- has never existed!