Astronomy Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2746
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Item ABOVE THE CLOUDS: 1-D MODELING OF OBSERVATIONS OF TIDALLY LOCKED EXTRASOLAR WORLDS(2019) Afrin Badhan, Mahmuda; Deming, L. Drake; Domagal-Goldman, Shawn D.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Unique and exotic planetary environments give us an opportunity to understand how planetary systems form and evolve over their lifetime, by placing our planetary system in the context of vastly different extrasolar systems. With orbital separations a fraction of the Mercury-Sun distance, these close-in planets provide us with valuable insights regarding interactions between the stellar and planetary atmospheres. Further, observational biases actually allow such planets to be the first to be observed via transit spectroscopy. Observed spectrophotometric signatures from transit measurements can reveal spectrally active species in a planet’s atmosphere. Present observational technologies can also shed light on the atmosphere’s structure and dynamics. Future missions will allow us to constrain these properties with unprecedented accuracy, and are also being designed to observe increasingly smaller, cooler and less extreme planets. The eventual goal, after all, is to identify a world like our own. To interpret the observations with any certainty, however, we must build robust atmospheric models that sufficiently factor both physical and chemical processes expected in those atmospheres. 3-D climate modeling has shown that tidally-locked Earth-like planets, at the inner edge of M dwarf habitable zones, may retain water-vapor-rich stratospheres. However, flaring M dwarfs have strong UV activity, which may photodisassociate H2O. Using synthetical stellar UV within a 1-D photochemical model, I assess whether water vapor loss driven by high stellar UV would affect its detectability in JWST/MIRI transmission spectroscopy. I pseudo-couple a 3-D climate model to our 1-D model to achieve this. In a follow-up study, I also compute 125 additional atmospheric states by varying the Earth-like planet’s orbital distance (thus moistness) and methane production rates. I check for and quantify the simultaneous presence of detectable ozone and methane in an otherwise abiotic anoxic atmosphere. I have also implemented techniques to robustly quantify atmospheric properties of hot Jupiters from data-driven retrievals and built a versatile template for hot Jupiter atmospheres within our 1-D photochemical modeling tool, which was previously only valid for cool rocky worlds. I sketch out a plan for using this work towards mapping non-equilibrated (non-LTE) emissions from methane in the upper atmospheres of observable giants.Item Shocks and Cold Fronts in Galaxy Clusters --- Probing the Microphysics of the Intracluster Medium(2018) Wang, Qian; Mushotzky, Richard; Markevitch, Maxim; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Shocks and cold fronts in galaxy clusters, the largest gravitationally bound systems in the universe, are astrophysical laboratories where we can study the microphysics of the intracluster medium (ICM), a very hot ($T\sim10^7$--$10^8$~K) plasma. Being the main baryon content of galaxy clusters, the ICM plays an important role in mediating the energy cascade from gravitational collapse during cosmological structure formation. It is also intricately linked to the evolution of the galaxies within. The scientific enquiries concerning the ICM range from fundamental physics questions to cosmological measurements. In this dissertation, I demonstrate probing ICM microphysics by studying deep X-ray observations of two galaxy clusters, A520 and A2142. For A520, tests for thermal conduction, electron--ion equilibration timescale, and particle acceleration at the shock were carried out. For A2142, a test for the effective viscosity was performed using two apparent Kelvin-Helmholtz eddies along its southern cold front. Other interesting features were discovered and analyzed, such as a low gas fraction subcluster in the A520 outskirts, and X-ray deficient channels that could be plasma depletion sheets in both clusters.Item Accretion Physics Through the Lens of the Observer: Connecting Fundamental Theory with Variability from Black Holes(2018) Hogg, James Andrew; Reynolds, Christopher S; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Variability is a generic feature of accretion onto black holes. In both X-ray binaries and active galactic nuclei, variability is observed on nearly all accessible timescales and across the entire electromagnetic spectrum. On different timescales and at different wavelengths it has unique signatures that can be used to characterize the accretion processes generating the emission and probe the accretion disks, which would otherwise be impossible. Despite having been observed for over fifty years, interpreting this variability is difficult. Simple phenomenological models have been used to explain the behaviors and geometries of the observed accretion disk, but they have yet to be rigorously tested in a full magnetohydrodynamic framework. In this dissertation we use high-resolution numerical models to investigate: (1) ``propagating fluctuations" in mass accretion rate that give rise to the nonlinear signatures of accretion on viscous timescales, (2) the dynamics of truncated accretion disks which are invoked to explain the spectral variation of outbursting X-ray binaries and the bifurcation of AGN accretion states, and (3) the large-scale magnetic dynamo behavior in thick and thin accretion disks. We find that the structured variability readily seen in the light curves from accreting black holes (i.e. log-normal flux distributions, linear relations between the RMS and the flux, and radial coherence) quickly and naturally grows from the MRI-driven turbulence and that these properties translate into photometric variability. For the first time, we identify the large-scale magnetic dynamo as the source of the low-frequency modulations of the disk stress that cause this structure. We introduce a bistable cooling law into hydrodynamic and magnetohydrodynamic simulations to study the manifestation of a truncated accretion disk in each regime. We find that rather than a truncation edge, the transition is better described by a ``truncation zone" when the angular momentum transport and heating is governed by MRI-driven turbulence instead of a true viscosity. Additionally, we find that the hot gas in the simulation buoyantly rises in a gentle outflow and eventually fills the entire volume, instead of simply being confined to the innermost region. The outflow interacts with the disk body and enhances the magnetic stresses, which could produce stronger quasiperiodic variability. Finally, we conduct an investigation of the large-scale magnetic dynamo using a suite of four global magnetohydrodynamic disk simulations with scaleheight ratios of $h/r=\{0.05, 0.1, 0.2, 0.4\}$. Most notably, the organization that is prevalent in accretion disk simulations and described as a ``butterfly pattern" does not occur when $h/r \ge 0.2$, despite the dynamo action still operating efficiently.Item ATMOSPHERIC CHARACTERIZATION OF GIANT EXOPLANETS IN EXTREME ENVIRONMENTS(2017) Wilkins, Ashlee Noelle; Deming, Leo D; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The study of planets around other stars has entered a science-rich era of characterization, in which detailed information about individual planets can be inferred from observations beyond discovery and confirmation, which only yield bulk properties like mass or radius. Characterization probes more revealing quantities such as chemical abundances, albedo, and temperature/pressure profiles, allowing us to address larger questions of planet formation mechanisms, planetary evolution, and, eventually, presence of biosignature gases. The primary method for characterization of close-in planets is transit spectroscopy. My dissertation comprises transiting exoplanet case studies using the Hubble Space Telescopes Wide-Field Camera-3 (HST/WFC3) as a tool of exoplanet characterization in a near-infrared band dominated by broad water absorption. Much of my efforts went toward a characterization of the WFC3 systematic effects that must be mitigated to extract the incredibly small (tens to 200 parts per million) signals. The case study subjects in this dissertation are CoRoT-2b (in emission), WASP-18b (in transmission and emission), and HATS-7b (in transmission), along with some partial/preliminary analyses of HAT-p-3b and HD 149026b (both in transmission). I also present an analysis of transit timing of WASP-18b with HST and other observatories as another clue to its evolution as a close-in, extremely massive planet purported to be spiraling in to its host star. The five planets range from super Neptunes to Super-Jupiter in size/mass. The observability of such planets – i.e. giants across a continuum of mass/size in extreme local environments close to their respective host stars, – is a unique opportunity to probe planet formation and evolution, as well as atmospheric structures in a high-irradiation environment. This genre of observations reveal insights into aerosols in the atmosphere; clouds and/or hazes can significantly impact atmospheric chemistry and observational signatures, and the community must better understand the phenomenon of aerosols in advance of the next generation of space observatories, including JWST and WFIRST. In conducting these case studies as part of larger collaborations and HST observing campaigns, my work aids in the advancement of exoplanet atmosphere characterization from single, planetby-planet, case studies, to an understanding of the large, hot, gaseous planets as a population.Item Optical Time Domain and Radio Imaging Analyses of the Dynamic Hearts of AGN(2017) Smith, Krista Lynne; Mushotzky, Richard; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Active galactic nuclei (AGN) are among the most extreme objects in the universe: galaxies with a central supermassive black hole feeding on gas from a hot accretion disk. Despite their potential as powerful tools to study topics ranging from relativity to cosmology, they remain quite mysterious. In the first portion of this thesis, we explore how an AGN may influence the formation of stars in its host galaxy. Using high-resolution 22 GHz radio imaging of an X-ray selected sample of radio-quiet AGN, we find that the far-infrared radio correlation for normal star forming galaxies remains valid within a few hundred parsecs of the central engine. Because the core flux is often spatially isolated from star formation, we can also determine that the radio emission in radio-quiet AGN is consistent with both coronal and disk-jet coupling models. Finally, we find that AGN with jet-like radio morphologies have suppressed star formation, possibly indicating ongoing feedback. The second portion of this thesis uses optical AGN light curves to study the physics of accretion. The Kepler spacecraft produces groundbreaking light curves, but its fixed field of view only contained a handful of known AGN. We conduct an X-ray survey of this field, yielding 93 unique X-ray sources identified by optical follow-up spectroscopy as a mixture of AGN and stars. For the AGN, we spectroscopically measure black hole masses and accretion rates. We then analyze a sample of 22 Kepler AGN light curves. We develop a customized pipeline for AGN science with Kepler, a necessary step since the initial data was optimized for the unique goal of exoplanet detection. The light curves display an astonishing variety of behaviors in a new regime of optical variability inaccessible with previous facilities. We find power spectral slopes inconsistent with the damped random walk model, characteristic variability timescales, correlations of variability properties with physical parameters, and bimodal flux distributions possibly consistent with passing obscuring material. We also conclude that this regime of optical variability is not produced by simple X-ray reprocessing. Finally, we explain how this work supports future robust accretion studies with upcoming large timing surveys.Item MAGNETOHYDRODYNAMIC SIMULATIONS OF BLACK HOLE ACCRETION(2017) Avara, Mark James; Reynolds, Christopher S; McKinney, Jonathan; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Black holes embody one of the few, simple, solutions to the Einstein field equations that describe our modern understanding of gravitation. In isolation they are small, dark, and elusive. However, when a gas cloud or star wanders too close, they light up our universe in a way no other cosmic object can. The processes of magnetohydrodynamics which describe the accretion inflow and outflows of plasma around black holes are highly coupled and nonlinear and so require numerical experiments for elucidation. These processes are at the heart of astrophysics since black holes, once they somehow reach super-massive status, influence the evolution of the largest structures in the universe. It has been my goal, with the body of work comprising this thesis, to explore the ways in which the influence of black holes on their surroundings differs from the predictions of standard accretion models. I have especially focused on how magnetization of the greater black hole environment can impact accretion systems.Item Numerical Simulations of Granular Physics in the Solar System(2017) Ballouz, Ronald; Richardson, Derek C; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Granular physics is a sub-discipline of physics that attempts to combine principles that have been developed for both solid-state physics and engineering (such as soil mechanics) with fluid dynamics in order to formulate a coherent theory for the description of granular materials, which are found in both terrestrial (e.g., earthquakes, landslides, and pharmaceuticals) and extra-terrestrial settings (e.g., asteroids surfaces, asteroid interiors, and planetary ring systems). In the case of our solar system, the growth of this sub-discipline has been key in helping to interpret the formation, structure, and evolution of both asteroids and planetary rings. It is difficult to develop a deterministic theory for granular materials due to the fact that granular systems are composed of a large number of elements that interact through a non-linear combination of various forces (mechanical, gravitational, and electrostatic, for example) leading to a high degree of stochasticity. Hence, we study these environments using an N-body code, pkdgrav, that is able to simulate the gravitational, collisional, and cohesive interactions of grains. Using pkdgrav, I have studied the size segregation on asteroid surfaces due to seismic shaking (the Brazil-nut effect), the interaction of the OSIRIS-REx asteroid sample-return mission sampling head, TAGSAM, with the surface of the asteroid Bennu, the collisional disruptions of rubble-pile asteroids, and the formation of structure in Saturn's rings. In all of these scenarios, I have found that the evolution of a granular system depends sensitively on the intrinsic properties of the individual grains (size, shape, sand surface roughness). For example, through our simulations, we have been able to determine relationships between regolith properties and the amount of surface penetration a spacecraft achieves upon landing. Furthermore, we have demonstrated that this relationship also depends on the strength of the local gravity. By comparing our numerical results to laboratory experiments and observations by spacecraft we can begin to understand which microscopic properties (i.e., grain properties) control the macroscopic properties of the system. For example, we can compare the mechanical response of a spacecraft to landing or Cassini observations of Saturn's ring to understand how the penetration depth of a spacecraft or the complex optical depth structure of a ring system depends on the size and surface properties of the grains in those systems.Item A Statistical Characterization of the Atmospheres of Sub-Saturn Planet Candidates in the Kepler Archive(2016) Sheets, Holly Ann; Deming, L. Drake; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Exoplanet atmospheric characterization is still in its early stages. Large surveys like the Kepler Mission provide thousands of planet candidates, but follow-up observations to characterize the individual candidates are often difficult to obtain. In this thesis, I develop a method to detect small atmospheric signals in Kepler’s planet candidate light curves by averaging light curves for multiple candidates with similar orbital and physical characteristics. I also consider two applications of this method: at secondary eclipse, to determine the average albedo of groups of planet candidates, and near transit, to determine whether on average the planets have cloud-free, low- mean-molecular-weight atmospheres, or cloudy/hazy/high-mean-molecular-weight atmospheres. This approach allows the measurement of properties that are un- measurable for candidates individually, because of their low signal-to-noise, and it prevents biasing of the results by false positives (candidates that are not actually planets) and outliers by not depending on only a few measurable candidates. I first develop the method and apply it to the secondary eclipses of 32 close-in Kepler planet candidates between 1 and 6 R⊕ with short cadence data available, in order to determine their average albedo. I then adapt the method to the long cadence data, accounting for the effects of the longer integration time. The increase in the number of candidates available in long cadence allows for finer radius groupings of 1 to 2 R⊕, 2 to 4 R⊕, and 4 to 6 R⊕. The short cadence average includes 6,238 individual eclipses, while the long cadence averages contain 80,492 eclipses from 56 candidates in the 1 to 2 R⊕ bin, 22,677 eclipses from 38 candidates in the 2 to 4 R⊕ bin, and 4,572 eclipses from 16 candidates in the 4 to 6 R⊕ bin. In both studies, I find that these planet candidates are generally dark, though there are bright outliers like Kepler-10b, and I discuss the implications of these results for understanding the atmospheres of these planets. Finally, I apply the method to Kepler planet candidates in short cadence near transit, looking for a brief brightening due to light that is refracted through the atmospheres of the planets and directed toward the observer just before and just after transit. Refracted light is strongest in planetary atmospheres that are cloud-free and have a low mean molecular weight. Preliminary results suggest this strong refraction effect is not present in the selected group of 10 candidates with radii between 0.8 and 3 R⊕, but I begin to develop a more detailed model and sketch out future plans to improve the model and to continue testing for the presence of refracted light with greater sensitivity.Item Gamma-Ray Studies of Stellar Graveyards: Fermi-LAT Observations of Supernova Remnants and Spatially Extended Emission(2016) Cohen, Jamie M.; Hays, Elizabeth; Miller, Coleman; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)When a massive star explodes as a supernova, it injects a huge amount of energy into its surroundings. The resultant expanding blast-wave and its interaction with the surrounding medium is known as a supernova remnant (SNR). The shock created by the supernova event is believed to be the primary accelerator of cosmic rays (CRs) in our Galaxy. While SNRs are observable across the electromagnetic spectrum, studying the gamma-ray emission from these sources is crucial in understanding the origin of CRs and acceleration processes acting therein. Recent advances in gamma-ray astronomy present new opportunities to study the aftermath of stellar explosions at gamma-ray energies. In 2008 the Fermi Gamma-Ray Space Telescope was launched into orbit and, with its unmatched gamma-ray resolution, has opened up a new window on the high-energy sky. In this thesis, I present new work using data from the primary instrument on the Fermi observatory, the Large Area Telescope (LAT), to study both individual SNRs as well as the population of remnants observable by the LAT, with a focus on searching for spatially extended emission from these remnants. To uniformly determine the high-energy properties of SNRs, I developed an automated method to systematically characterize the gamma-ray emission in a region of the sky. Applying this method to the locations of several hundred radio-observed SNRs, we classified 30 gamma-ray sources as likely being associated with SNRs. Our results, combined with archival radio, X-ray, and TeV observations, serve to challenge previously sufficient, simple gamma-ray SNR emission models. I also present a study of the sources detected above 50 GeV, focusing on those lying in the Galactic plane. 31 sources were shown to be significantly spatially extended with 5 of those being newly detected. Finally, I present a dedicated analysis of one of the 5 newly detected extended sources. I determined that the extended GeV emission likely originated from the shock of SNR G150.3+4.5. Combined with archival radio and X-ray data, I consider several possible origin scenarios, including one in which the SNR may be one of the youngest, closest gamma-ray SNRs detected by the LAT.Item BETTII: A pathfinder for high angular resolution observations of star-forming regions in the far-infrared(2016) Rizzo, Maxime Jean; Mundy, Lee G; Rinehart, Stephen A; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, we study clustered star formation in nearby star clusters and discuss how high angular resolution observations in the far-infrared regime could help us understand these important regions of stellar birth. We use the increased angular resolution from the FORCAST instrument on the SOFIA airborne observatory to study 10 nearby star-forming regions, and discuss the physical properties of sources in these regions that we can infer from radiative transfer modeling using these new observations. We discuss the design of BETTII, a pathfinder balloon-borne interferometer which will provide significantly better angular resolution in the far-infrared regime, and pave the way for future space-borne observatories. We elaborate on the details of BETTII's core technique, called Double-Fourier interferometry, and how to accurately compute the sensitivity of instruments which use this technique. Finally, we show our design and implementation results of the control and attitude estimation system for the BETTII payload, which poses unique challenges as an interferometer on a balloon platform.