An asteroseismic test of diffusion theory in white dwarfs

¹ High Altitude Observatory, National Center for Atmospheric Research, PO Box 3000, Boulder, CO 80307-3000, USA e-mail: travis@hao.ucar.edu
² Department of Astronomy, University of Texas, Austin, TX 78712, USA
³ Southwestern University, 1001 E. University Avenue, Georgetown, TX 78626, USA
⁴ Korea Astronomy Observatory, Daejeon 305-348, Korea
⁵ Institut für Astronomie, Universität Wien, Türkenschanzstraße 17, 1180 Wien, Austria

Received: 23 December 2004
Accepted: 6 February 2005

Abstract

The helium-atmosphere (DB) white dwarfs are commonly thought to be the descendants of the hotter PG 1159 stars, which initially have uniform He/C/O atmospheres. In this evolutionary scenario, diffusion builds a pure He surface layer which gradually thickens as the star cools. In the temperature range of the pulsating DB white dwarfs ( K) this transformation is still taking place, allowing asteroseismic tests of the theory. We have obtained dual-site observations of the pulsating DB star CBS 114, to complement existing observations of the slightly cooler star GD 358. We recover the 7 independent pulsation modes that were previously known, and we discover 4 new ones to provide additional constraints on the models. We perform objective global fitting of our updated double-layered envelope models to both sets of observations, leading to determinations of the envelope masses and pure He surface layers that qualitatively agree with the expectations of diffusion theory. These results provide new asteroseismic evidence supporting one of the central assumptions of spectral evolution theory, linking the DB white dwarfs to PG 1159 stars.