Physics and Astronomy electronic theses and dissertations (MU)
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The items in this collection are the theses and dissertations written by students of the Department of Physics and Astronomy. Some items may be viewed only by members of the University of Missouri System and/or University of Missouri-Columbia. Click on one of the browse buttons above for a complete listing of the works.
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Item Motion of test particles around a black hole in the Hubble universe(University of Missouri--Columbia, 2025) Jayswal, Vishal; Kopeikin, Sergei M.We consider the Friedmann universe as a cosmological background manifold, with a point mass m embedded within it. The presence of this mass perturbs the gravitational field (spacetime geometry) of the Friedmann universe. To accurately describe this perturbation, it is necessary to construct a metric that seamlessly integrates elements of both the Schwarzschild and FLRW metrics. We explore two distinct approaches to formulating this model: one introduced by McVittie [1], which employs global coordinates to solve Einstein’s field equations, and another developed by Lasenby and his collaborators [2], which utilizes the tetrad formalism. For a spatially flat universe 𝑘 = 0, the expressions for density and pressure derived from McVittie's and Lasenby's approaches coincide under an appropriate coordinate transformation. However, for an open 𝑘 = -1 or closed 𝑘 = +1 perturbed Friedmann universe, they do not. Likewise, in the case of a spatially flat universe 𝑘 = 0, the Lasenby metric can be mapped to McVittie's metric through the same coordinate transformation. However, for open and closed universes, the Lasenby metric cannot be transformed into McVittie’s metric. Our objective is to analyze the mathematical differences between these two physically equivalent approaches and figured out whether the problem of a point mass in a cosmological manifold admits a unique solution. To this end, we establish a correspondence between the components of various geometric objects—such as the metric tensor, Ricci tensor, and energy-momentum tensor—defined in both coordinate and tetrad bases. We reformulate Einstein's field equations as a system of first-order partial differential equations for the tetrad components of a time-dependent, spherically symmetric gravitational field. As a subsequent step, we investigate the orbital motion of a test particle in the gravitational field of a massive body (that might be a black hole) with mass m, placed within an expanding cosmological manifold described by the McVittie metric. We introduce local coordinates attached to the massive body to eliminate nonphysical, coordinatedependent effects associated with Hubble expansion. The resultant equations of motion for the test particle are analyzed using the method of osculating elements, along with the time-averaging technique. We demonstrate that the orbit of the test particle is not affected by cosmological expansion up to terms of the second order in the Hubble parameter. However, cosmological expansion induces orbital precession over time and modifies the frequency of the mean orbital motion. We show that the direction of orbital precession depends on both the Hubble parameter and the deceleration parameter of the universe. Finally, we provide numerical estimates for the rate of orbital precession over time due to cosmological expansion in several astrophysical systems.Item A study of PAHs in the universe : organic molecules tracing astrophysical structures, grain growth, and distribution(University of Missouri--Columbia, 2025) Mentzer, Charles Edmond; Li, AigenPolycyclic Aromatic Hydrocarbons (PAHs) are a widespread and chemically significant component of the interstellar medium, recognized by their distinct infrared emission bands at 3.3, 6.2, 7.7, 8.6, and 11.3 μm. These emission features—originally known as the Unidentified Infrared Emission (UIE) bands—are sensitive to the physical and chemical conditions of their environments, making PAHs powerful tracers of local radiation fields, grain growth, and molecular evolution. This dissertation presents a comprehensive study of PAHs and aliphatic hydrocarbons across a wide range of astrophysical systems, from circumstellar envelopes to high-redshift galaxies, using data from Spitzer, the Very Large Telescope (VLT), and the James Webb Space Telescope (JWST). Through a combination of spectral modeling and spatially resolved analysis, I characterize the size, ionization state, and chemical composition of organic molecules and evaluate their diagnostic potential in tracing grain growth, radiation environments, and the structure of their host systems. The first part of this dissertation focuses on the presence of PAHs in the UV-poor environments surrounding cool carbon-rich stars. Using archival data from the Spitzer Space Telescope, I investigate seven such sources: IRAS Z02229+6208, IRAS 20000+3239, IRAS 22272+5435, IRAS 22574+6609, IRAS 23304+6147, W Orionis, and IRAS 13416–624. Despite the relatively low effective temperatures of these stars (Teff < 6000 K), strong PAH emission bands are observed in their circumstellar envelopes. I model the vibrational excitation of PAHs under these UV-deficient conditions and demonstrate that visible photons are sufficient to excite PAHs to levels consistent with the observed UIE features. For each source, I derive the characteristic PAH sizes, charge fractions, and total PAH mass, and discuss the implications for PAH formation, survivability, and chemical processing in these evolved environments. In the second part, I transition to the protoplanetary disk (PPD) phase of star formation, where PAHs play a role in disk chemistry and structure. Using high spatial resolution VISIR-NEAR longslit spectroscopic observations from the VLT, I analyze the PAH emission bands at 8.6 and 11.3 μm in the disks surrounding two Herbig Ae stars: HD 97048 and HD 169142. These bands, extending out to ∼100–200AU from the central stars, offer insight into the spatial distribution and evolution of PAH properties within the disks. By modeling these features, I derive radial profiles of PAH size, ionization state, and mass, revealing how local disk conditions—such as gas temperature, electron density, and radiation intensity—affect the charge state and composition of PAHs. This analysis underscores the utility of PAHs as diagnostics of disk structure and evolutionary stage. The third component of this work examines merging galaxies with prominent starburst and AGN activity, using JWST/MIRI integral field spectroscopy. I focus on NGC 3256 and VV 114, both nearby luminous infrared galaxies (LIRGs) characterized by merging cores and active star-forming arms. In each system, I select multiple regions with a mix of physical and spatial characteristics and use the spatially resolved 6.2, 7.7, and 11.3 μm PAH bands to characterize the ionization fraction and representative size of the PAH population. Despite the intense radiation fields expected near an AGN, we find surprisingly small PAH molecules surviving within 100 pc of the nuclei. Additionally, I estimate the energy density parameter U for each region, bounded below by the local mass and above by the requirement for single-photon heating, to trace the evolution of the local radiation field. These results suggest that even in AGN-dominated environments, PAHs persist and can be used to trace energetic feedback. Expanding the spectral coverage to shorter wavelengths, I employ JWST/NIRSpec integral field spectroscopy to probe the 3.3 and 3.4 μm bands in NGC 3256 and VV 114 across 64 regions each, with 20-40 pc apertures. These features are associated with aromatic and aliphatic hydrocarbons, respectively, and provide critical insight into grain chemistry and ice interactions. Both galaxies exhibit clear aliphatic emission bands closer to their AGN centers than previously anticipated, along with enhanced 3.4-3.6 μm emission features that had been too weak to detect prior to JWST. The presence of strong water ice extinction across these systems points to heavily obscured cores. We quantify the aliphatic-to-aromatic ratios across the spatial regions and conclude that the AGN influence extends to ∼180 pc in both galaxies, reshaping our understanding of where complex organic molecules can persist in such extreme environments. Finally, I turn to a high-redshift system, J0749+2255 at z = 2.17, which hosts two merging AGN cores and active star formation. Using rest-frame JWST/MIRI data targeting the 3.3 and 3.4 μm features, I analyze 42 spatial regions to study the distribution and abundance of aliphatic hydrocarbons in the early universe. This work confirms, for the first time, the widespread presence of aliphatic compounds at high redshift, marking J0749+2255 as the most distant galaxy with confirmed aliphatic emission to date. I derive regional aliphatic fractions and discuss the implications for dust processing and organic grain evolution in the early stages of galaxy assembly. Together, these studies demonstrate that PAHs and aliphatic hydrocarbons serve as sensitive tracers of the local radiation environment, grain evolution, and feedback processes across a vast range of astrophysical contexts—from cool stars and protoplanetary disks to starburst galaxies and the high-redshift universe. With the unparalleled spatial and spectral resolution of JWST, combined with legacy observations from Spitzer and the VLT, this dissertation reveals the resilience, variability, and diagnostic power of organic molecules in shaping and tracing the universe’s most dynamic environments.Item Time-dependent exciton dynamics : insights from 1- and 2-dimensional model systems(University of Missouri--Columbia, 2024) Williams, Jared Robert; Ullrich, CarstenTime-Dependent Density-Functional Theory (TDDFT) has become a promising alternative to the Bethe-Salpeter Equation (BSE) in terms of the calculation of optical spectra, including excitoinic features, due to its significantly lower computational cost. TDDFT brings with it the advantage of, in principle, access to the Time-Dependent (TD) exciton wave function and the ability to study ultra-fast TD phenomena. One promising family of Exchange & Correlation (XC) functionals for capturing excitonic effects in TDDFT are the Long-Range Corrected (LRC) functionals. In order to test the LRC functionals, we have written programs to do calculations in both a One-Dimensional (1D) and Two-Dimensional (2D) model systems. We used the 1D model system to explore avenues of TD exciton wave function visualization to facilitate the study and understanding of the TD dynamics of excitons. Likewise, we used the 2D model system to study various methods of correction to the LRC in hopes of better capturing excitonic effects and improving stability for Real-Time (RT) calculations. We show examples of exciton wave functions visualized with various techniques, including waterfall plots and heat maps. We have also used our 1D model system to explore how RT calculations performed using TDDFT can be used to gain insight into novel systems, as exemplified by our study of charge-transfer excitations. Using our 2D model system we have explored two possible avenues of correction to the LRC: adding a 'counter' term to the XC functional and 'generalizing' the differential equation which defines the LRC. The first avenue we explored is the addition of a new term to the XC potential, explicitly constructed to cancel out the error produced by the LRC's violation of the zero-force theorem, leading to the introduction of spurious, nonphysical, internal forces. This approach, in the end, does not seem to make a substantial impact on the optical absorption spectra or the stability of the LRC. The second avenue, using the so-called "Proca" equation, adds terms to the equation of motion in order to stabilize the calculations-again with limited success. We have successfully demonstrated in our model systems that the RT methods for TDDFT are promising tools for the exploration of TD phenomena in electronic systems. We have also explored these techniques using some illustrative situations and avenues for improving the accuracy and stability of these calculations.Item Theoretical and computational modeling of the interactions of peptides and proteins with lipid bilayer surfaces(University of Missouri--Columbia, 2024) Smith, Ryan; Kosztin, IoanThis dissertation combines computational and theoretical modeling with experimental data to advance our understanding of peptide-lipid interactions, leveraging coarse-grained (CG) molecular dynamics (MD) simulations and atomic force microscopy (AFM)-based force spectroscopy to elucidate these essential biological processes. We investigated the impact of polypeptide length on membrane interactions using a series of short, homologous peptides, revealing a linear dependence of free energy barriers and intrinsic dissociation rates on peptide length. Another study examined the effect of primary structure by varying guest residue species and positions within host-guest pentapeptides, finding significant effects of terminal residues on interactions with zwitterionic and anionic lipid bilayers and identifying multiple dissociation pathways. Additionally, we applied and compared several computational techniques for analyzing AFM force spectroscopy data, proposing machine learning methods for improved analysis. These findings suggest that peptide-membrane interactions might be more complex than previously understood, often requiring multiple dissociation pathways. Finally, we extended CG MD simulations to study a large peripheral membrane protein, providing insights into interactions at the lipid membrane surface from its dominant binding regions. This study highlights limitations in current methods and suggests alternative approaches for studying complete proteins.Item Structural and electronic properties of hybrid halide perovskites : insights into chemical vapor deposited films(University of Missouri--Columbia, 2024) Burns, Randy Daniel; Guha, SuchismitaIn the past several years, hybrid perovskite semiconductors that have been incorporated into solar cells have experienced an extremely rapid increase in power conversion efficiency that now rivals traditional silicon based devices. Hybrid organic-inorganic (HOIP) halide perovskites combine inorganic, metallic materials such as lead and tin, with organic compound groups such as methylammonium or formamidinium along with halogen atoms such as iodine, chlorine, or bromine. The combination of ions based off of those elements culminates in thin films with a variety of fortuitous characteristics. HOIP materials exhibit high absorption coefficients, tunable band gaps, extremely high defect tolerance in comparison to silicon, and allow for low-temperature solution processing and vapor deposition growth methods. Unfortunately, the main obstacle preventing mass commercialization of hybrid perovskite solar cells is stability. Moisture, heat, and light induce substantial stress onto perovskite materials and cause irreversible, destructive degradation. As such, research is now focused on developing perovskite thin films that can withstand external stimuli that may cause degradation mechanisms to occur in perovskite based devices. Many methods exist that help improve perovskite stability including modifications to the growth technique to improve the intrinsic sample quality, composition engineering of the perovskite material, encapsulation with polymers or glass, post-treatment annealing or vapor assisted defect passivation, and many more. This work primarily focuses on developing and describing precisely how chemical vapor deposition (CVD) as a growth technique, in contrast to many solution-based methods such as spin coating, greatly impacts the downstream characteristics of the perovskite thin films by enhancing stability, favorable charge transport, and shifting the structural phase diagram of the material. Furthermore, this work demonstrates that many characteristics of perovskite materials that are conventionally thought of as intrinsic are actually greatly dependent, in practice, on the growth environment that ultimately produces the thin films as opposed to idealized models which assume very low defect densities. This work demonstrates air stable perovskite thin films through CVD that exist in the ideal cubic phase at room temperature with increased electronic transport. High hydrostatic pressure was used as a probe to elucidate the phase diagram and optical pressure response of 3D perovskites. Additionally, the CVD method can be augmented to produce chlorine incorporated perovskite thin films that exhibit a single phase structure across a wide temperature range where chlorine acts as a defect passivator. More exotic forms of the material such as 2D Ruddlesden-Popper (RP) were grown with improved columnar alignment in comparison to spin coated films. Finally, a post-deposition electron-beam irradiation treatment is explored and shows how low doses of radiation of CVD grown films can improve optical and electrical performance.