Nucleosynthesis Predictions For Intermediate-Mass Asymptotic Giant Branch Stars: Comparison To Observations Of Type I Planetary Nebulae

Type I planetary nebulae (PNe) have high He/H and N/O ratios and are thought to be descendants of stars with initial masses of similar to 3-8 M(circle dot). These characteristics indicate that the progenitor stars experienced proton-capture nucleosynthesis at the base of the convective envelope, in addition to the slow neutron capture process operating in the He-shell (the s-process). We compare the predicted abundances of elements up to Sr from models of intermediate-mass asymptotic giant branch (AGB) stars to measured abundances in Type I PNe. In particular, we compare predictions and observations for the light trans-iron elements Se and Kr, in order to constrain convective mixing and the s-process in these stars. A partial mixing zone is included in selected models to explore the effect of a (13)C pocket on the s-process yields. The solar-metallicity models produce enrichments of [(Se, Kr)/Fe] less than or similar to 0.6, consistent with Galactic Type I PNe where the observed enhancements are typically less than or similar to 0.3 dex, while lower metallicity models predict larger enrichments of C, N, Se, and Kr. O destruction occurs in the most massive models but it is not efficient enough to account for the less than or similar to 0.3 dex O depletions observed in some Type I PNe. It is not possible to reach firm conclusions regarding the neutron source operating in massive AGB stars from Se and Kr abundances in Type I PNe; abundances for more s-process elements may help to distinguish between the two neutron sources. We predict that only the most massive (M greater than or similar to 5 M(circle dot)) models would evolve into Type I PNe, indicating that extra-mixing processes are active in lower-mass stars (3-4 M(circle dot)), if these stars are to evolve into Type I PNe.