H ingestion into He-burning convection zones in super-AGB stellar models as a potential site for intermediate neutron-density nucleosynthesis

Abstract

We investigate the evolution of super-AGB thermal pulse (TP) stars for a range of metallicities (Z) and explore the effect of convective boundary mixing (CBM). With decreasing metallicity and evolution along the TP phase, the He-shell flash and the third dredge-up (TDU) occur closer together in time. After some time (depending upon the CBM parameterisation), efficient TDU begins while the pulse-driven convection zone (PDCZ) is still present, causing a convective exchange of material between the PDCZ and the convective envelope. This results in the ingestion of protons into the convective He-burning pulse. Even small amounts of CBM encourage the interaction of the convection zones leading to transport of protons from the convective envelope into the He layer. H-burning luminosities exceed $10^9$ (in some cases $10^{10}$) ${\mathrm{L}}_{\odot }$. We also calculate models of dredge-out in the most massive super-AGB stars and show that the dredge-out phenomenon is another likely site of convective-reactive H-${}^{12}$C combustion. We discuss the substantial uncertainties of stellar evolution models under these conditions. Nevertheless, the simulations suggest that in the convective-reactive H-combustion regime of H ingestion the star may encounter conditions for the intermediate neutron capture process (i process). We speculate that some CEMP-s/r stars could originate in i-process conditions in the H-ingestion phases of low-Z SAGB stars. This scenario would however suggest a very low electron-capture supernova rate from super-AGB stars. We also simulate potential outbursts triggered by such H-ingestion events, present their light curves and briefly discuss their transient properties.

arXiv commentAccepted for publication in MNRAS; 17 pages, 12 figures, 3 tables

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