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Topic

The Impact of Extra Mixing in Low-Mass Stars on Presolar Grain Abundance Predictions

Department of Physics and Astronomy

Date & location

• Thursday, December 4, 2025
• 12:00 P.M.
• Virtual Defence

Examining Committee

Supervisory Committee

• Dr. Iris Dillmann, Department of Physics and Astronomy, ӣ��Ӱ�� (Co-Supervisor)
• Dr. Falk Herwig, Department of Physics and Astronomy, UVic (Co-Supervisor)

External Examiner

• Dr. Nan Liu, Institute for Astrophysical Research, Boston University

Chair of Oral Examination

• Dr. Terri Lacourse, Department of Biology, UVic

Abstract

Presolar oxide grains record isotopic signatures of stellar nucleosynthesis, offering direct 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, patterns attributed to slow extra mixing between the convective envelope and the hydrogen-burning shell—known as cool bottom processing (CBP). This work presents a self-consistent stellar evolution model of secular CBP in low-mass asymptotic giant branch (AGB) stars that reproduces the observed oxygen isotopic ratios in Group 2 grains and the surface abundances inferred from spectroscopy of similar-mass AGB stars.

The results show that CBP can be naturally self-limiting: an increase in the mean molecular weight gradient as small as Δ𝜇 ≈ 𝜇_CE × 10⁻⁵ is sufficient to halt further mixing. This stabilizing feedback supports the interpretation of CBP as a slow, low-energy circulation that preserves stellar structure. Variations in the depth and efficiency of mixing were found to span the full range of observed isotopic ratios. A nuclear-physics sensitivity study identified the ¹⁸O(p,𝛼)¹⁴N reaction as the dominant source of uncertainty, with smaller but measurable contributions from the ¹⁸O(p,𝛼)¹⁵N and ¹⁶O(p,𝛾)¹⁷F reactions.