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

Jones, S.; Ritter, C.; Herwig, F.; Fryer, C.; Pignatari, M.; Bertolli, M. G.; Paxton, B.

Abstract

We investigate the evolution of super-AGB (SAGB) 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 parametrization), 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 10⁹ (in some cases 10¹⁰) L⊙. We also calculate models of dredge-out in the most massive SAGB stars and show that the dredge-out phenomenon is another likely site of convective-reactive H-¹²C 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 SAGB stars. We also simulate potential outbursts triggered by such H ingestion events, present their light curves and briefly discuss their transient properties.

Citation

Jones, S., Ritter, C., Herwig, F., Fryer, C., Pignatari, M., Bertolli, M. G., & Paxton, B. (2016). H ingestion into He-burning convection zones in super-AGB stellar models as a potential site for intermediate neutron-density nucleosynthesis. Monthly notices of the Royal Astronomical Society, 455(4), 3848-3863. https://doi.org/10.1093/mnras/stv2488

• Journal Article Type: Article
• Acceptance Date: Oct 23, 2015
• Online Publication Date: Dec 3, 2015
• Publication Date: Feb 1, 2016
• Deposit Date: Apr 1, 2016
• Publicly Available Date: Apr 1, 2016
• Journal: Monthly notices of the Royal Astronomical Society
• Print ISSN: 0035-8711
• Electronic ISSN: 1365-2966
• Publisher: Oxford University Press
• Peer Reviewed: Peer Reviewed
• Volume: 455
• Issue: 4
• Pages: 3848-3863
• DOI: https://doi.org/10.1093/mnras/stv2488
• Keywords: Stars: abundances, Stars: AGB and post-AGB, Stars: evolution, Stars: interiors
• Public URL: https://hull-repository.worktribe.com/output/434656
• Publisher URL: http://mnras.oxfordjournals.org/content/455/4/3848
• Additional Information: This article has been accepted for publication in Monthly notices of the Royal Astronomical Society. ©2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.