Dissipative Dynamics of Stars, Planets, and Black Holes

Author: Ma, Linhao

Year: 2024

Degree: Dissertation (Ph.D.)

Advisor: Fuller, James

Committee Members: Hopkins, Philip F.; Howard, Andrew W.; Phinney, E. Sterl; Steidel, Charles C.; Fuller, James

Option: Physics

DOI: 10.7907/9j1c-ec50

Abstract

In this dissertation, I present a series of theoretical works on two important dissipative mechanisms in the universe, namely dynamical friction and tidal dissipation. I discuss the physics of these processes, and investigate how they will affect the dynamical evolution of stars, planets, and black holes.

I develop a new sub-grid dynamical friction estimator based on the discrete nature of N-body simulations. This estimator avoids the ambiguously defined quantities in Chandrasekhar's dynamical friction formula. I test the estimator in the GIZMO code, and find that it agrees well with high-resolution simulations where dynamical friction is fully captured. The additional computational cost with this estimator is negligible, making it an efficient and implementable solution to sub-grid dynamical friction modeling.

I study the dynamics of massive black hole seeds in high-redshift galaxies. I analyze the direct N-body integration of seed black hole trajectories with high-resolution cosmological simulations, and calculate the dynamics of randomly generated test particles in post-processing with dynamical friction. I find that seed black holes less massive than 100 million solar-masses (i.e. all but the already-supermassive seeds) cannot efficiently sink to the galactic center in typical high-redshift galaxies. This finding provides new constraints on the formation models of super-massive black holes in the most distant galaxies.

I study the effects of tidal resonance locking for exoplanet systems, in which the planet locks into resonance with a tidally excited stellar gravity mode. I find that due to nonlinear mode damping, resonance locking in Sun-like stars likely only operates for low-mass planets, but in stars with convective cores it can likely operate for all planetary masses. The orbital decay timescale with resonance locking is typically comparable to the star's main-sequence lifetime, corresponding to a wide range in effective stellar quality factor, depending on the planet's mass and orbital period. I make predictions for several individual systems and examine the orbital evolution resulting from both resonance locking and nonlinear wave dissipation.

I investigate the tidal spin-up of subdwarf B (sdB) star binaries. I directly calculate the tidal excitation of internal gravity waves in realistic sdB stellar models, and integrate the coupled spin-orbit evolution of sdB binaries. I find that for canonical sdB binaries, the transitional orbital period below which they could reach tidal synchronization in the sdB lifetime is approximately 0.2 days, with weak dependence on the companion masses. This value is very similar to the tidal synchronization boundary evident from observations.

I investigate the scenario of tidal spin-up of Wolf-Rayet-black-hole binaries, which is a possible way to form the fast-rotating black holes observed from gravitational wave events. I directly calculate the tidal excitation of oscillation modes in Wolf-Rayet star models, determining the tidal spin-up rate, and integrating the coupled spin-orbit evolution for Wolf-Rayet-black-hole binaries. I find that for short-period orbits and massive Wolf-Rayet stars, the tidal interaction is mostly contributed by standing gravity modes, in contrast to Zahn's model of traveling waves which is frequently assumed in the literature. I show that tidal synchronization is rarely reached in Wolf-Rayet-black-hole binaries, and the resulting black hole spins are less than 0.4 for all but the shortest period binaries.

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