Mixing Uncertainties in Low-Metallicity AGB Stars: The Impact on Stellar Structure and Nucleosynthesis

Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Eötvös Loránd Research Network (ELKH), Konkoly Thege M. út 15-17, 1121 Budapest, Hungary

Department of Physics & Astronomy, University of Victoria, Victoria, BC V8P 5C2, Canada

⁴

Joint Institute for Nuclear Astrophysics, Center for the Evolution of the Elements, Michigan State University, 640 South Shaw Lane, East Lansing, MI 48824, USA

Author to whom correspondence should be addressed.

^†

The NuGrid Collaboration, http://www.nugridstars.org.

Academic Editor: Sylvia Ekström

Universe 2021, 7(2), 25; https://doi.org/10.3390/universe7020025

Received: 22 December 2020 / Revised: 21 January 2021 / Accepted: 22 January 2021 / Published: 26 January 2021

(This article belongs to the Special Issue AGB Stars: Element Forges of the Universe)

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Abstract

The slow neutron-capture process (s-process) efficiency in low-mass AGB stars (1.5 < M/M

_{⊙}

< 3) critically depends on how mixing processes in stellar interiors are handled, which is still affected by considerable uncertainties. In this work, we compute the evolution and nucleosynthesis of low-mass AGB stars at low metallicities using the MESA stellar evolution code. The combined data set includes models with initial masses M

_{i n i}

_{⊙} = 2

and 3 for initial metallicities

Z = 0.001

and

0.002

. The nucleosynthesis was calculated for all relevant isotopes by post-processing with the NuGrid mppnp code. Using these models, we show the impact of the uncertainties affecting the main mixing processes on heavy element nucleosynthesis, such as convection and mixing at convective boundaries. We finally compare our theoretical predictions with observed surface abundances on low-metallicity stars. We find that mixing at the interface between the He-intershell and the CO-core has a critical impact on the s-process at low metallicities, and its importance is comparable to convective boundary mixing processes under the convective envelope, which determine the formation and size of the ¹³C-pocket. Additionally, our results indicate that models with very low to no mixing below the He-intershell during thermal pulses, and with a ¹³C-pocket size of at least ∼3 × 10

^{- 4}

_{⊙}

, are strongly favored in reproducing observations. Online access to complete yield data tables is also provided. View Full-Text

Keywords: evolved stars; stellar evolution; stellar interiors; stellar mixing; nucleosynthesis

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MDPI and ACS Style

Battino, U.; Lederer-Woods, C.; Cseh, B.; Denissenkov, P.; Herwig, F. Mixing Uncertainties in Low-Metallicity AGB Stars: The Impact on Stellar Structure and Nucleosynthesis. Universe 2021, 7, 25. https://doi.org/10.3390/universe7020025

AMA Style

Battino U, Lederer-Woods C, Cseh B, Denissenkov P, Herwig F. Mixing Uncertainties in Low-Metallicity AGB Stars: The Impact on Stellar Structure and Nucleosynthesis. Universe. 2021; 7(2):25. https://doi.org/10.3390/universe7020025

Chicago/Turabian Style

Battino, Umberto, Claudia Lederer-Woods, Borbála Cseh, Pavel Denissenkov, and Falk Herwig. 2021. "Mixing Uncertainties in Low-Metallicity AGB Stars: The Impact on Stellar Structure and Nucleosynthesis" Universe 7, no. 2: 25. https://doi.org/10.3390/universe7020025

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Mixing Uncertainties in Low-Metallicity AGB Stars: The Impact on Stellar Structure and Nucleosynthesis

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