Application of a theory and simulation-based convective boundary mixing model for AGB star evolution and nucleosynthesis
- Department of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel (Switzerland)
- E.A. Milne Centre for Astrophysics, Department of Physics and Mathematics, University of Hull, HU6 7RX (United Kingdom)
- Department of Physics and Astronomy, University of Victoria, Victoria, BC, V8P5C2 (Canada)
- Keele University, Keele, Staffordshire ST5 5BG (United Kingdom)
- Department of the Geophysical Sciences and Chicago Center for Cosmochemistry, Chicago, IL 60637 (United States)
- Department of Physics and Astronomy at Uppsala University, Regementsvagen 1, Box 516, SE-75120 Uppsala (Sweden)
- Kavli Institute for Theoretical Physics and Department of Physics, Kohn Hall, University of California, Santa Barbara, CA 93106 (United States)
The s-process nucleosynthesis in Asymptotic giant branch (AGB) stars depends on the modeling of convective boundaries. We present models and s-process simulations that adopt a treatment of convective boundaries based on the results of hydrodynamic simulations and on the theory of mixing due to gravity waves in the vicinity of convective boundaries. Hydrodynamics simulations suggest the presence of convective boundary mixing (CBM) at the bottom of the thermal pulse-driven convective zone. Similarly, convection-induced mixing processes are proposed for the mixing below the convective envelope during third dredge-up (TDU), where the {sup 13}C pocket for the s process in AGB stars forms. In this work, we apply a CBM model motivated by simulations and theory to models with initial mass M = 2 and M=3 M{sub ⊙}, and with initial metal content Z = 0.01 and Z = 0.02. As reported previously, the He-intershell abundances of {sup 12}C and {sup 16}O are increased by CBM at the bottom of the pulse-driven convection zone. This mixing is affecting the {sup 22}Ne(α, n){sup 25}Mg activation and the s-process efficiency in the {sup 13}C-pocket. In our model, CBM at the bottom of the convective envelope during the TDU represents gravity wave mixing. Furthermore, we take into account the fact that hydrodynamic simulations indicate a declining mixing efficiency that is already about a pressure scale height from the convective boundaries, compared to mixing-length theory. We obtain the formation of the {sup 13}C-pocket with a mass of ≈10{sup −4} M{sub ⊙}. The final s-process abundances are characterized by 0.36<[s/Fe]<0.78 and the heavy-to-light s-process ratio is −0.23<[hs/ls]<0.45. Finally, we compare our results with stellar observations, presolar grain measurements and previous work.
- OSTI ID:
- 22868807
- Journal Information:
- Astrophysical Journal, Vol. 827, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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