TY - JOUR
T1 - Evolution and nucleosynthesis of helium-rich asymptotic giant branch models
AU - Shingles, Luke J.
AU - Doherty, Carolyn L.
AU - Karakas, Amanda I.
AU - Stancliffe, Richard J.
AU - Lattanzio, John C.
AU - Lugaro, Maria
N1 - Publisher Copyright:
© 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.
PY - 2015/10/27
Y1 - 2015/10/27
N2 - 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 ~4-5M⊙ with Y = 0.40. We also model the production of trans-Fe elements via the slow neutron-capture process (sprocess). 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.
AB - 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 ~4-5M⊙ with Y = 0.40. We also model the production of trans-Fe elements via the slow neutron-capture process (sprocess). 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.
KW - Nuclear reactions, nucleosynthesis, abundances
KW - Stars: AGB and post-AGB
KW - Stars: evolution
KW - Stars: interiors
UR - http://www.scopus.com/inward/record.url?scp=84945299182&partnerID=8YFLogxK
U2 - 10.1093/mnras/stv1489
DO - 10.1093/mnras/stv1489
M3 - Article
SN - 0035-8711
VL - 452
SP - 2804
EP - 2821
JO - Monthly Notices of the Royal Astronomical Society
JF - Monthly Notices of the Royal Astronomical Society
IS - 3
ER -