Evolution and nucleosynthesis of helium-rich asymptotic giant branch models
Jul 2, 2015
18 pages
Published in:
- Mon.Not.Roy.Astron.Soc. 452 (2015) 3, 2804-2821
- Published: Sep 21, 2015
e-Print:
- 1507.00741 [astro-ph.SR]
View in:
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Abstract: (Oxford University Press)
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 intermediate-mass 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 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 per cent 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 per cent 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.Note:
- 21 pages, 16 figures, accepted for publication by MNRAS. Stellar yields included as online data tables
- model: production
- star
- n: capture
- nucleosynthesis
- temperature
- efficiency
- helium
- thermal
References(149)
Figures(16)