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

F. Herwig^1,2 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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