Astronomy and Astrophysics – Astrophysics
Scientific paper
Jun 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002a%26a...388..320w&link_type=abstract
Astronomy and Astrophysics, v.388, p.320-325 (2002)
Astronomy and Astrophysics
Astrophysics
11
Stars: Atmospheres, Stars: Interiors, Stars: Variables: General, Stars: White Dwarfs
Scientific paper
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 Msun, 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{Montgomery.Winget00}) 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. Based on observations collected at the European Southern Observatory, Paranal, Chile (ESO Programme 67.D-0047).
Koester Detlev
Montgomery Michael H.
Winget Don E.
Wolff Burkhard
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