Seismic Probes of Stellar Mergers and Magnetism

Author: Rui, Nicholas Zhao

Year: 2025

Degree: Dissertation (Ph.D.)

Advisor: Fuller, James

Committee Members: Phinney, E. Sterl; Fuller, James; El-Badry, Kareem J.; Kasliwal, Mansi M.; Most, Elias R.

Option: Physics

DOI: 10.7907/9stk-9462

Abstract

Stellar pulsations can do what most other astrophysical observables cannot: directly probe internal stellar properties. This thesis consolidates work investigating how stellar oscillation modes are affected by two common but "noncanonical" pieces of stellar physics: mergers and magnetism.

The earlier chapters develop "seismic stellar merger genealogy," the application of seismology to the discovery of stellar merger remnants. In Chapter II, I show that red giants which have engulfed close, main-sequence companions possess unusual gravity-mode period spacings, indicating their binary origin. I identify two dozen promising merger remnant candidates in archival Kepler data, roughly consistent with expected stellar merger rates. In Chapter III, I study the evolution and properties of the red-giant-like stars which result from coalescences of accreting helium-core white dwarf systems. These merger remnants display distinctive seismic and chemical properties, particularly during the core helium-burning phase as the result of an especially violent helium flash.

The later chapters develop "seismic stellar magnetometry," the application of seismology to the measurement of stellar magnetic fields. In Chapter IV, I calculate the morphology of high-radial-order gravity modes under the influence of strong magnetic fields. The eigenfunctions exhibit two morphological features at which energy dissipation may be strong, in agreement with the suppressed dipole modes observed in many red giants. In Chapter V, I apply the same method to calculate the gravity-mode period spacing pattern under a strong magnetic field. The perturbative theory developed for weak fields underestimates the true frequency shifts to gravity modes caused by strong magnetic fields. In Chapter VI, I model the behavior of stochastic pulsators whose magnetic fields are strong enough to misalign their pulsations from the rotation axis. Even in the presence of stochasticity, the light curves of such oblique pulsators indefinitely retain some phase information in a way that can be used to identify them. In Chapter VII, I place upper bounds on the near-surface magnetic fields of a sample of white dwarfs based on the non-detection of magnetic features in their pulsation spectra. Although these constraints vary significantly with white dwarf structure and mode periods, they are consistently much stronger than the megagauss-scale magnetic fields to which spectroscopy is sensitive.

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