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The Evolutionary Pathways of Tidal Disruption Events: from Stars to Debris Streams, Accretion Disks, and Relativistic Jets

TitleThe Evolutionary Pathways of Tidal Disruption Events: from Stars to Debris Streams, Accretion Disks, and Relativistic Jets
Publication TypeThesis
Year of Publication2016
AuthorsCoughlin, ER
Academic DepartmentAstrophysical and Planetary Sciences
DegreePh.D.
Number of Pages295
Date Published06-2016
UniversityUniversity of Colorado
CityBoulder, CO
Abstract

Tidal disruption events, which occur when a star is destroyed by the gravitational field of a supermassive black hole, are unique probes of the inner regions of galaxies. In this thesis, we explore various stages of the tidal disruption process, in an attempt to relate the observable signatures of tidal disruption events to the properties of the disrupted star and the black hole. We use numerical techniques to study the long-term evolution of the debris streams produced from tidal disruption events, showing that they can be gravitationally unstable and, as a result of the instability, fragment into small-scale, localized clumps. The implications of this finding are discussed, and we investigate how the thermodynamic properties of the gas comprising the stream affect the nature of the instability. We derive an analytic model for the structure of tidally disrupted stellar debris streams, and we compare the predictions of our model to numerical results. We present a model for the accretion disk that forms from a tidal disruption event when the accretion rate surpasses the Eddington limit of the supermassive black hole, showing that these disks are puffed up into quasi-spherical envelopes that are threaded by bipolar relativistic jets. We compare the predictions of this model to observations of the jetted-tidal disruption event Swift J1644+57. Finally, we derive, from the relativistic Boltzmann equation, the general relativistic equations of radiation hydrodynamics in the viscous limit, which characterize the interaction between radiation and matter when changes in the fluid over the photon mean free path are small. Our results demonstrate that, in contrast to previous works, a radiation-dominated fluid does in fact possess a finite bulk viscosity and a correction to the co-moving energy density. Using the general relativistic equations of radiation hydrodynamics in the viscous limit, we present two models to describe the interaction between a relativistic jet launched during a tidal disruption event and its surroundings. These models show that regions of very large shear that arise between the fast-moving outflow and the surrounding envelope possess fewer scatterers and a harder photon spectrum, meaning that observers looking "down the barrel of the jet" infer vastly different properties of the outflow than those who look off-axis.