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

<p>Tidal disruption events, which occur when a star is destroyed by the gravitational field of a\&nbsp;<span style="line-height: 1.6em;">supermassive black hole, are unique probes of the inner regions of galaxies. In this thesis, we explore\&nbsp;</span><span style="line-height: 1.6em;">various stages of the tidal disruption process, in an attempt to relate the observable signatures of\&nbsp;</span><span style="line-height: 1.6em;">tidal disruption events to the properties of the disrupted star and the black hole. We use numerical\&nbsp;</span><span style="line-height: 1.6em;">techniques to study the long-term evolution of the debris streams produced from tidal disruption\&nbsp;</span><span style="line-height: 1.6em;">events, showing that they can be gravitationally unstable and, as a result of the instability, fragment\&nbsp;</span><span style="line-height: 1.6em;">into small-scale, localized clumps. The implications of this finding are discussed, and we\&nbsp;</span><span style="line-height: 1.6em;">investigate how the thermodynamic properties of the gas comprising the stream affect the nature\&nbsp;</span><span style="line-height: 1.6em;">of the instability. We derive an analytic model for the structure of tidally disrupted stellar debris\&nbsp;</span><span style="line-height: 1.6em;">streams, and we compare the predictions of our model to numerical results. We present a model for\&nbsp;</span><span style="line-height: 1.6em;">the accretion disk that forms from a tidal disruption event when the accretion rate surpasses the\&nbsp;</span><span style="line-height: 1.6em;">Eddington limit of the supermassive black hole, showing that these disks are puffed up into quasi-spherical\&nbsp;</span><span style="line-height: 1.6em;">envelopes that are threaded by bipolar relativistic jets. We compare the predictions of\&nbsp;</span><span style="line-height: 1.6em;">this model to observations of the jetted-tidal disruption event Swift J1644+57. Finally, we derive,\&nbsp;</span><span style="line-height: 1.6em;">from the relativistic Boltzmann equation, the general relativistic equations of radiation hydrodynamics\&nbsp;</span><span style="line-height: 1.6em;">in the viscous limit, which characterize the interaction between radiation and matter when\&nbsp;</span><span style="line-height: 1.6em;">changes in the\&nbsp;</span><span style="line-height: 1.6em;">fluid over the photon mean free path are small. Our results demonstrate that, in\&nbsp;</span><span style="line-height: 1.6em;">contrast to previous works, a radiation-dominated\&nbsp;</span><span style="line-height: 1.6em;">fluid does in fact possess a finite bulk viscosity\&nbsp;</span><span style="line-height: 1.6em;">and a correction to the co-moving energy density. Using the general relativistic equations of radiation\&nbsp;</span><span style="line-height: 1.6em;">hydrodynamics in the viscous limit, we present two models to describe the interaction between\&nbsp;</span><span style="line-height: 1.6em;">a relativistic jet launched during a tidal disruption event and its surroundings. These models show\&nbsp;</span><span style="line-height: 1.6em;">that regions of very large shear that arise between the fast-moving outfl</span><span style="line-height: 1.6em;">ow and the surrounding\&nbsp;</span><span style="line-height: 1.6em;">envelope possess fewer scatterers and a harder photon spectrum, meaning that observers looking\&nbsp;</span><span style="line-height: 1.6em;">"down the barrel of the jet" infer vastly different properties of the outfl</span><span style="line-height: 1.6em;">ow than those who look\&nbsp;</span><span style="line-height: 1.6em;">off-axis.</span></p>
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University of Colorado Boulder
Boulder, CO
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