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

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 Mini/M⊙=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 M⊙, are strongly favored in reproducing observations. Online access to complete yield data tables is also provided.

Magnetic-buoyancy-induced mixing in AGB Stars: fluorine nucleosynthesis at different metallicities

“…Nonetheless, we expect a non-trivial mass-metallicity dependence of the mixing triggered by magnetic buoyancy (as for other type of mixing, see e.g. Joyce & Chaboyer 2018;Battino et al 2021). In particular, the toroidal field strength is determined by the differential rotation profile established in the He-intershell and which amplifies the magnetic field via dynamo action (see Section 4).…”

Magnetic-buoyancy-induced mixing in AGB stars: Fluorine nucleosynthesis at different metallicities

“…Vescovi et al (2020) found that most of the heavy-element isotope ratios measured in presolar SiC grains from AGB stars are consistently explained by stellar AGB models computed with a unique choice for the toroidal field strength and the initial buoyant velocity, namely B ϕ = 5×10 4 G and u p = 5×10 −5 cm s −1 . Nonetheless, we expect a nontrivial mass-metallicity dependence of the mixing triggered by magnetic buoyancy (as for other type of mixing; see, e.g., Joyce & Chaboyer 2018;Battino et al 2021). In particular, the toroidal field strength is determined by the differential rotation profile established in the He-intershell that amplifies the magnetic field through dynamo action (see Sect.…”

Exploring the s-process history in the Galactic disk: Cerium abundances and gradients in Open Clusters from the OCCAM/APOGEE sample

“…The formation of an extended 13 C pocket induced by mixing processes can increase Ce production (e.g. Battino et al 2021) and may explain its overabundance in the young open clusters, as pointed out by Maiorca et al (2011).…”