Although we know that many supernovae are aspherical, the exact nature of their geometry is undetermined. Because all the supernovae we observe are too distant to be resolved, the ejecta structure can't be directly imaged, and asymmetry must be inferred from signatures in the spectral features and polarization of the supernova light. The empirical interpretation of this data, however, is rather limited--to learn more about the detailed supernova geometry, the oretical modeling must been undertaken. One expects the geometry to be closely tied to the explosion mechanism and the progenitor star system, both of which are still under debate. Studying the 3-dimensional structure of supernovae should therefore provide new break throughs in our understanding. The goal of this thesis is to advance new techniques for calculating radiative transfer in 3-dimensional expanding atmospheres, and use them to study the flux and polarization signatures of aspherical supernovae. We develop a 3-D Monte Carlo transfer code and use it to directly fit recent spectropolarimetric observations, as well as calculate the observable properties of detailed multi-dimensional hydrodynamical explosion simulations. While previous theoretical efforts have been restricted to ellipsoidal models, we study several more complicated configurations that are tied to specific physical scenarios. We explore clumpy and toroidal geometries in fitting the spectropolarimetry of the Type Ia supernova SN 2001el. We then calculate the observable consequences of a supernova that has been rendered asymmetric by crashing into a nearby companion star. Finally, we fit the spectrum of a peculiar and extraordinarily luminous Type Ic supernova. The results are brought to bear on three broader astrophysical questions: (1) What are the progenitors and the explosion processes of Type Ia supernovae? (2) What effect does asymmetry have on the observational diversity of Type Ia supernovae, and hence their use in cosmology? (3) And, what are some of the physical properties of Type Ic supernovae, believed to be associated with gamma-ray bursts?

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## Scholarly Works (13 results)

This thesis presents numerical calculations designed to understand aspects of the feeding of, and feedback from, super-massive black holes at the centers of galaxies. The first portion of the thesis describes the development of radiative transfer tools used to address these problems. I present a description of how the Monte Carlo technique can be used to solve the radiative transfer equation, and I demonstrate a coupling of the transfer solution with the equations of hydrodynamics and statistical equilibrium. I next present two major applications of these ideas. The first is to quantify the effects of radiative feedback in active galactic nuclei in the form of radiation pressure on dust at the center of a gas-rich galaxy. The second is a calculation of the spectral energy distributions and optical spectral line strengths emitted during the tidal disruption of a star by a massive black hole. In so doing, I help to answer a number of puzzling questions relating to such disruptions, such as whether the lack of hydrogen emission in their spectra can be the result of radiative transfer effects rather than a lack of hydrogen in the disrupted star.

This thesis presents models of the radiative signatures of two unusual electromagnetic transients: radioactive transients ("kilonovae") associated with the merger of a neutron star with a second neutron star or a black hole, and broad-lined Type Ic supernovae. The first portion of the thesis focuses on improving inputs for radiation transport simulations of kilonovae. I first address opacity. I present new calculations of the opacities of elements synthesized in the kilonova ejecta, and carry out radiation transport simulations of kilonovae using these new opacities. Next, I develop detailed models of the radioactive heating of the kilonova ejecta and explore its effects on kilonova light curves. These two projects advanced our understanding of the expected emission from kilonovae, and will inform efforts to find and analyze these elusive transients. The final part of this thesis considers broad-lined Ic supernovae. I use multidimensional hydrodynamical and radiation transport simulations to investigate the validity of the single-engine model for broad-lined superonva Ic/gamma-ray burst systems. I demonstrate that a jet engine injected into a stripped-envelope progenitor star can produce both a supernova with broad spectral features and a long-duration gamma-ray burst.

In the favored progenitor scenario, Type Ia supernovae (SNe Ia) arise from a white dwarf accreting material from a non-degenerate companionstar. Soon after the white dwarf explodes, the ejected supernova material engulfs the companion star; two-dimensional hydrodynamicals imulations by Marietta et al. (2001) show that, in the interaction, the companion star carves out a conical hole of opening angle 30-40 degrees in the supernova ejecta. In this paper we use multi-dimensional Monte Carlo radiative transfer calculations to explore the observable consequences of an ejecta-hole asymmetry. We calculate the variation of the spectrum, luminosity, and polarization with viewing angle for the aspherical supernova near maximum light. We find that the supernova looks normal from almost all viewing angles except when one looks almost directly down the hole. In the latter case, one sees into the deeper, hotter layers of ejecta. The supernova is relatively brighter and has a peculiar spectrum characterized by more highly ionized species, weaker absorption features, and lower absorption velocities. The spectrum viewed down the hole is comparable to the class of SN 1991T-like supernovae. We consider how the ejecta-hole asymmetry may explain the current spectropolarimetric observations of SNe Ia, and suggest a few observational signatures of the geometry. Finally, we discuss the variety currently seen in observed SNe Ia and how an ejecta-hole asymmetry may fit in as one of several possible sources of diversity.

This dissertation presents solutions to some problems associated with using strongly gravitationally lensed supernovae (gLSNe) to measure the cosmological parameters. Chief among these are new methods for finding gLSNe and extracting their time delays in the presence of microlensing. The first of these results increased the expected gLSN yields of the Zwicky Transient Facility and the Large Synoptic Survey Telescope, upcoming wide-field optical imaging surveys, by an order of magnitude. The latter of these results involved performing simulations of radiation transport in supernova atmospheres. These simulations provided evidence that some Type Ia supernovae come from sub-Chandrasekhar mass progenitors.

Though fundamental to our understanding of stellar, galactic, and cosmic evolution, the stellar explosions known as supernovae (SNe) remain mysterious. We know that mass loss and mass transfer are central processes in the evolution of a star to the supernova event, particularly for thermonuclear Type Ia supernovae (SNe Ia), which are in a close binary system. The circumstellar environment (CSE) contains a record of the mass lost from the progenitor system in the centuries prior to explosion and is therefore a key diagnostic of SN progenitors. Unfortunately, tools for studying the CSE are specialized to stellar winds rather than the more complicated and violent mass-loss processes hypothesized for SN Ia progenitors.

This thesis presents models for constraining the properties of a CSE detached from the stellar surface. In such cases, the circumstellar material (CSM) may not be observed until interaction occurs and dominates the SN light weeks or even months after maximum light. I suggest we call SNe with delayed interaction SNe X;n (i.e. SNe Ia;n, SNe Ib;n). I per- formed numerical hydrodynamic simulations and radiation transport calculations to study the evolution of shocks in these systems. I distilled these results into simple equations that translate radio luminosity into a physical description of the CSE. I applied my straightfor- ward procedure to derive upper limits on the CSM for three SNe Ia: SN 2011fe, SN 2014J, and SN 2015cp. I modeled interaction to late times for the SN Ia;n PTF11kx; this led to my participation in the program that discovered interaction in SN 2015cp. Finally, I ex- panded my simulations to study the Type Ib;n SN 2014C, the first optically-confirmed SN X;n with a radio detection. My SN 2014C models represent the first time an SN X;n has been simultaneous modeled in the x-ray and radio wavelengths.

We show that the shape of P-Cygni line profiles of photospheric phase supernova can be analytically inverted to extract both the optical depth and source function of the line -- i.e. all the physical content of the model for the case when the Sobolev approximation is valid. Under various simplifying assumptions, we derive formulae that give S(r) and tau(r) in terms of derivatives of the line flux with respect to wavelength. The transition region between the minimum and maximum of the line profile turns out to give especially interesting information on the optical depth near the photosphere. The formulae give insights into the relationship between line shape and physical quantities that may be useful in interpreting observed spectra and detailed numerical calculations.