Convective proton and 3He ingestion into helium burning: Nucleosynthesis during a post-AGB thermal pulse

F. Herwig1,2 and N. Langer3

1Universität Potsdam, Institut für Physik, Astrophysik
3Dept. of Physics and Astronomy, University of Victoria, Canada
3University Utrecht, Utrecht, The Netherlands

A thermal pulse during the post-AGB phase of stellar evolution may lead to a unique mode of light element nucleosynthesis. The stage is set by the ingestion of the unprocessed envelope material into the hot He-flash convection zone below. If the temperature is sufficiently large and the 12C abundance high enough (e.g. T8 > 0.8, X(12C) round 0.4 and X(H) round 1 . 10-3) protons react faster with 12C and form 13C than destroying 7Be. The latter forms by alpha-capture of 3He after an initial reduction of the 3He abundance to about 3 . 10-5X(1He) by the ppI reaction 3He(3HE,2p)1He (for T8 round 1). All 3He is burned within minutes to weeks depending on the temperature. 7Be is now present at about the previously mentioned level of 3He. Its further fate is determined by the reactions 7Be(e-, nu)7Li and the alpha-capture reactions of 7Be and 7Li. These captures lead to the production of 11B which in turn is finally destroyed by 11B(alpha, n)11N. The details of this mechanism of light element production in real stars is expected to be fairly dependent on the description of mixing.

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