Is the helium in the variable DB white dwarfs ³He?^*

¹ Institut für Theoretische Physik und Astrophysik, Universität Kiel, 24098 Kiel, Germany e-mail: wolff, koester@astrophysik.uni-kiel.de
² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
³ Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA, UK e-mail: mikemon@ast.cam.ac.uk
⁴ Department of Astronomy, University of Texas at Austin, Austin, TX 78712, USA e-mail: dew@astro.as.utexas.edu

Corresponding author: D. Koester, koester@astrophysik.uni-kiel.de

Received: 12 February 2002
Accepted: 29 March 2002

Abstract

Asteroseismological analysis of the Whole Earth Telescope observations of the prototype variable DB white dwarf GD358 indicates a possible composition interface at a depth of about 10^-6 , which has usually been assumed to be the He/C transition zone. Such a small He layer is in contradiction with canonical evolution theory. Recently Montgomery & Winget ([CITE]) proposed that the transition zone is in fact the transition between the isotopes ³He and ⁴He, which are separated by the same sedimentation process separating hydrogen and helium in the DA white dwarfs. Therefore, the visible surface helium should be ³He. We have tested this hypothesis spectroscopically on another variable DB using the known relative isotopic shifts of up to 0.5 Å. The spectra are consistent with a pure ⁴He atmosphere, making a ³He/⁴He interface a very unlikely explanation for the mode spectrum of GD358.