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Topic

Impact of 3D Stellar Macrophysics: Gigayear White Dwarf Cooling Uncertainties and Minutes-to-Hours IGW Signatures in Massive Star Variability

Department of Physics and Astronomy

Date & location

• Thursday, August 14, 2025
• 12:30 P.M.
• Clearihue Building, Room B017

Examining Committee

Supervisory Committee

• Dr. Falk Herwig, Department of Physics and Astronomy, ßÉßɱ¬ÁÏ (Supervisor)
• Dr. Pavel Kovtun, Department of Physics and Astronomy, UVic (Member)

External Examiner

• Dr. Daniel Lecoanet, Department of Engineering Sciences and Applied Mathematics, Northwestern University

Chair of Oral Examination

• Dr. Sandra Gibbons, School of Exercise Science, Physical and Health Education, UVic

Abstract

Three-dimensional stellar physics affects observable stellar phenomena across different timescales. This thesis studies these effects through two investigations: gigayear-scale uncertainties in white dwarf cooling that affect galactic age determinations, and minutes-to-hours internal gravity wave signatures that produce variability in massive star light curves. These studies show how 3D physical processes, often neglected in one-dimensional stellar models, introduce uncertainties that limit our ability to extract reliable information from stellar observations.

White dwarfs serve as cosmic clocks for determining ages of stellar populations. Current practice reports cooling ages with uncertainties from observational errors in temperature and mass measurements, neglecting uncertainties in cooling models themselves. We expected that uncertainties in core composition profiles would dominate cooling age errors. However, investigation of twelve model parameters using MESA shows that envelope hydrogen and helium layer thicknesses contribute more to cooling age uncertainties than core composition variations. For a 0.6 𝑀_⊙ white dwarf, model uncertainties reach 0.8 Gyr at 4000 K, comparable to observational uncertainties. This sensitivity analysis shows how one-dimensional parameter studies can identify physical processes that require three-dimensional investigations to understand their impact on stellar evolution. This 1D investigation identifies key sources of systematic uncertainty in white dwarf age determinations, providing guidance for future observational and theoretical studies. The insights gained from such parameter studies inform the design of targeted 3D simulations that focus on the most physically significant processes identified through the sensitivity analysis.

Massive main-sequence stars show stochastic low-frequency variability in their light curves, but the physical mechanisms remain debated. Competing theories propose core convection-driven internal gravity waves versus subsurface convection as the source. 3D PPMstar simulations of a 25 𝑀_⊙ zero-age main-sequence star enable comparison between theoretical predictions and observational power spectra. Our simulations reproduce spectral characteristics that are qualitatively and quantitatively similar to observations, with two orders of magnitude power variation across frequencies. Controlled experiments isolating different stellar regions show that internal gravity waves excited at subsurface convective boundaries, rather than core convection, dominate the low-frequency excess. This finding resolves a question in massive star asteroseismology and validates the capability of 3D simulations to model observable stellar phenomena.

These investigations show how computational stellar physics can identify and quantify uncertainties that limit astronomical applications. The white dwarf study provides uncertainty estimates for galactic chronometry, while the massive star simulations establish a validated framework for interpreting asteroseismic observations. Both works show the importance of understanding model limitations in stellar physics applications.