Super and Massive AGB Stars: II - Nucleosynthesis and Yields - Z=0.02, 0.008 and 0.004.

Doherty, Carolyn and Gil-Pons, Pilar and Lau, Herbert H. B. and Lattanzio, John C. and Siess, Lionel (2014) Super and Massive AGB Stars: II - Nucleosynthesis and Yields - Z=0.02, 0.008 and 0.004. MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 437. pp. 195-214. ISSN 0035-8711

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Abstract

We have computed detailed evolution and nucleosynthesis models for super and massive AGB stars over the mass range 6.5 - 9.0 M⊙ in divisions of 0.5 M⊙ with metal- licities Z=0.02, 0.008 and 0.004. These calculations, in which we find third dredge-up and hot bottom burning, fill the gap between existing low and intermediate-mass AGB star models and high mass star models that become supernovae. For the considered metallicities, the composition of the yields is largely dominated by the thermodynamic conditions at the base of the convective envelope rather than by the pollution aris- ing from third dredge up. We investigate the effects of various uncertainties, related to the mass-loss rate, mixing length parameter, and the treatment of evolution after the envelope instability that develops near the end of the (Super)AGB phase. Vary- ing these parameters alter the yields mainly because of their impact on the amount of third dredge up enrichment, and to a lesser extent on the hot bottom burning conditions. Our models produce significant amounts of 4He, 7Li (depending on the mass-loss formulation) 13C, 14N, 17O, 23Na, 25Mg, as well the radioactive isotope 26Al in agreement with previous investigation. In addition our results show enrichment of 22Ne, 26Mg and 60Fe, as well as a substantial increase in our proxy neutron capture species representing all species heavier than iron. These stars may provide important contributions to the Galaxy’s inventory of the heavier Mg isotopes, 14N, 7Li and 27Al.

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