The impact of extra mixing in low-mass stars on explaining the isotopic compositions of presolar grains
Date
2025
Authors
Cockshutt, Maeve
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Presolar oxide grains preserve isotopic signatures of stellar nucleosynthesis in oxygen-rich stellar environments, providing stringent constraints on mixing and burning processes in evolved stars. Among these, Group 2 oxygen-rich grains exhibit enhanced 17O/16O ratios and severe 18O depletion, signatures widely attributed to slow extra mixing between the convective envelope and the hydrogen-burning shell, known as cool bottom processing (CBP). The physical origin and self-limiting nature of CBP have remained poorly constrained.
This work presents a self-consistent stellar evolution model of CBP operating in low-mass asymptotic giant branch (AGB) stars, implemented directly within the stellar structure. The model reproduces the full range of oxygen isotopic ratios observed in Group 2 presolar grains and is consistent with spectroscopic measurements of AGB stars of similar mass.
By evolving CBP concurrently with the stellar structure, this approach captures the development of a stabilizing mean molecular weight gradient. CBP is inherently self-limiting: an increase in the mean molecular weight gradient as small as ∆μ ≈ μCE × 10−5 is sufficient to completely suppress further circulation, supporting the interpretation of CBP as a slow, low-energy circulation confined to the radiative zone. Modest variations in the depth and efficiency of mixing reproduce the observed spread in isotopic ratios, highlighting both the robustness and degeneracy of CBP within this stabilizing framework.
A complementary nuclear-physics sensitivity study identified the 17O(p,α)14N reaction as the dominant source of uncertainty in predicted oxygen isotopic ratios, with smaller but measurable contributions from 18O(p,α)15N and 16O(p,γ)17F. For the 17O(p,α)14N reaction, simulations that best reproduce Group 2 presolar grain compositions preferentially favor either the JINA Reaclib rate, the indirect measurement of Sergi et al. (2020), or the lower bound from Rapagnani et al. (2025), while adoption of higher rates systematically degrades agreement with the grain data.
Comparisons with 3D hydrodynamics simulations suggest that wave-driven mixing enhanced by rapid rotation may plausibly approach the efficiency required by CBP, supporting it as a viable underlying mechanism for the parameterized extra mixing.
Description
Keywords
Presolar grains, Low-mass stars