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Topic

Convective-Reactive Nucleosynthesis in Oxygen-Carbon Shell Mergers

Department of Physics and Astronomy

Date & location

• August 14, 2025
• 9:00 A.M.
• Clearihue Building, Room B017

Examining Committee

Supervisory Committee

• Dr. Falk Herwig, Department of Physics and Astronomy, ßÉßɱ¬ÁÏ (Supervisor)
• Dr. Iris Dillmann, Department of Physics and Astronomy, UVic (Co-Supervisor)

External Examiner

• Prof. Almudena Arcones, Department of Physics, Technical University of Darmstadt

Chair of Oral Examination

• Dr. David Leitch, Department of Chemistry, UVic

Abstract

Many massive stars models at the end of their lives can have a merger between the convective O-burning shell and C-burning shells known as a O-C shell merger. During the merger, the material in the C-burning shell is ingested into the much hotter O-burning shell. Stellar models that have O-C shell mergers have been found to produce the stable isotopes of phosphorus, potassium, chlorine, and scandium which are otherwise not produced by stellar models to match the solar observations. O-C shell mergers are also a site for the production of the 𝑝 nuclei, a group of 35 𝑛-deficient stable isotopes produced by photo-induced reactions in this scenario. Part of the reason for this is that the O-burning shell is a convective-reactive environment, meaning that the timescales of the convective mixing and the nuclear reactions are comparable. Because of this, isotopes can be react at one location in the shell and the products can be transported to another location in the shell where they can be further processed. This changes which nucleosynthetic pathways are dominant and which isotopes are produced.

Stellar models calculate the stellar evolution and nucleosynthesis of the merger in 1D, but 3D hydrodynamic simulations significantly disagree with the 1D models at these late stages of stellar evolution. Convective velocities in 3D are much higher than 1D, the convective mixing profile features a downturn near the convective boundaries, the flow has large scale non-radial asymmetries, and the rate of ingesting C-shell material can be significantly lower. This is important for understanding the nucleosynthesis in the O-C shell merger, because if the convective velocities are different, then the timescale for mixing changes and the possible nucleosynthetic pathways can be altered.

In this thesis, I implement the insights from 3D hydrodynamic simulations into 1D stellar evolution models to determine how the macrophysical uncertainties in the O-C shell merger impact the nucleosynthesis in a convective-reactive environment, particularly for the 𝑝 nuclei. I also investigate the impact of nuclear reaction uncertainties on the nucleosynthesis of the 𝑝 nuclei during the O-C shell merger and this is influenced by the mixing conditions.