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Mixing Uncertainties in low-metallicity AGB Stars: the impact on Nucleosynthesis

Umberto Battino (University of Edinburgh)
When Dec 17, 2020
from 03:30 PM to 03:45 PM
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The s-process efficiency in low-mass AGB (1.5 < M/Msun < 3) critically depends on how mixing processes in stellar interiors are handled, which are still affected by considerable uncertainties. In this work we compute the evolution and nucleosynthesis of low-mass AGB stars at low metallicity using the MESA stellar code. The combined data set includes the initial masses Mzams/Msun = 2, 3 for Z = 0.001. The nucleosynthesis was calculated for all relevant isotopes in post-processing with the NuGrid mppnp code. Using these new models, we show the impact of the main mixing-processes affecting heavy elements nucleosynthesis, such as convection and mixing at convective boundaries. We finally compare our theoretical predictions with observed surface abundances on low metallicity AGB 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 mettallicity, and its importance is comparable to the 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 none mixing below the He-intershell during thermal pulses, and with a 13C-pocket size of at least ~3×10-4 Msun, are strongly favoured in reproducing observations. Finally, we show how increasing the size of available datasets, comprised of abundance measurements of stars in the metallicity range here considered, is critical to carry a solid validation of low-Z AGB stellar models.