Evolution and nucleosynthesis of helium-rich asymptotic giant branch models

Shingles, L. J. and Doherty, C. L. and Karakas, A. I. and Stancliffe, R. J. and Lattanzio, J. C. and Lugaro, Maria (2015) Evolution and nucleosynthesis of helium-rich asymptotic giant branch models. MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 452 (3). pp. 2804-2821. ISSN 0035-8711

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Abstract

There is now strong evidence that some stars have been born with He mass fractions as high as Y ≈ 0.40 (e.g., in ω Centauri). However, the advanced evolution, chemical yields, and final fates of He-rich stars are largely unexplored. We investigate the consequences of He-enhancement on the evolution and nucleosynthesis of intermediatemass asymptotic giant branch (AGB) models of 3, 4, 5, and 6 M with a metallicity of Z = 0.0006 ([Fe/H] ≈ −1.4). We compare models with He-enhanced compositions (Y = 0.30, 0.35, 0.40) to those with primordial He (Y = 0.24). We find that the minimum initial mass for C burning and super-AGB stars with CO(Ne) or ONe cores decreases from above our highest mass of 6 M to ∼ 4–5 M with Y = 0.40. We also model the production of trans-Fe elements via the slow neutron-capture process (s-process). He-enhancement substantially reduces the third dredge-up efficiency and the stellar yields of s-process elements (e.g., 90% less Ba for 6 M, Y = 0.40). An exception occurs for 3 M, where the near-doubling in the number of thermal pulses with Y = 0.40 leads to ∼ 50% higher yields of Ba-peak elements and Pb if the 13C neutron source is included. However, the thinner intershell and increased temperatures at the base of the convective envelope with Y = 0.40 probably inhibit the 13C neutron source at this mass. Future chemical evolution models with our yields might explain the evolution of s-process elements among He-rich stars in ω Centauri.

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