Volume 388, Number 1, June II 2002
|Page(s)||320 - 325|
|Section||Stellar structure and evolution|
|Published online||28 May 2002|
Is the helium in the variable DB white dwarfs 3He?*
Institut für Theoretische Physik und Astrophysik, Universität Kiel, 24098 Kiel, Germany e-mail: wolff, firstname.lastname@example.org
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: email@example.com
4 Department of Astronomy, University of Texas at Austin, Austin, TX 78712, USA e-mail: firstname.lastname@example.org
Corresponding author: D. Koester, email@example.com
Accepted: 29 March 2002
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.
Key words: stars: atmospheres / stars: interiors / stars: variables: general / stars: white dwarfs
© ESO, 2002
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