Astronomy
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Item Theoretical, Experimental, and Observational Studies of Iron X-ray Spectra: From the Laboratory to the Universe(2024) Grell, Gabriel Jonathan; Mushotzky, Richard; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The spectral lines of iron ions, particularly the dominant neon-like Fe XVII charge state, provide crucial diagnostics for the physical conditions of hot astrophysical plasmas in the X-ray regime. However, the diagnostic utility of these lines are hampered by significant discrepancies at the ~20% level between spectral observations, laboratory experiments, and theoretical calculations of the astrophysically important Fe XVII transitions, an issue that has been observed in numerous studies over several decades. Understanding the source of these discrepancies is critical for the improvement of both theoretical atomic models and laboratory experiment data on transition energies and cross sections of electron-ion processes, which themselves will be key for comparison to observations from X-ray spectroscopy missions such as XRISM, Line Emission Mapper (LEM), Arcus, and Athena. My dissertation encapsulates the main branches of X-ray astrophysics by focusing on the use of theoretical models and experimental measurements to further the diagnostic use, understanding, and interpretation of spectroscopic observations of iron transition lines. I modeled the effects of UV photoexcitation in O-type stars on a spectral line ratio of the Fe XVII 3s – 2p transitions in an attempt to explain an anomalous value found for the X-ray spectra of the O star ζ Puppis. I conjectured that the strong UV field of ζ Pup produces the observed ratio by depopulation of metastable 3s excited states, and that the ratio can potentially be used as an independent diagnostic of the radial distribution of X-ray-emitting plasma. Using the Flexible Atomic Code (FAC) collisional-radiative model to model the effect of UV photoexcitation on the Fe XVII lines, I compared the model calculations to archival spectra of coronal and hot stars from the Chandra HETGS and XMM-Newton RGS. The calculations showed that UV photoexcitation does not produce a sufficiently large dynamic range in the Fe XVII line ratio to explain the difference in the observed ratio between coronal stars and ζ Pup. I used FAC to compute steady-state populations of Fe XVII states and calculate cross sections for the dielectronic recombination (DR) and direct electron-impact excitation (DE) line formation channels of Fe XVII, and benchmarked the model predictions with experimental cross sections of Fe XVII resonances that were mono-energetically excited in an electron beam ion trap (EBIT) experiment. I extended the benchmark to all resolved DR and DE channels in the experimental dataset with a focus on the n ≥ 4 DR resonances, finding that the DR and DE absolute cross section predictions for the higher n complexes disagree considerably with experimental results when using the same methods as in previous works. However, agreement within ∼10% of the experimental results was achieved by an approach whereby I doubly convolve the predicted cross sections with both the spread of the electron-beam energy and the photon-energy resolution of the EBIT experiment. I also calculated rate coefficients from the experimental and theoretical cross sections, finding general agreement within 2σ with the rates found in the OPEN-ADAS atomic database. Circling back to the ζ Pup Fe XVII ratio, I probed the potential significance of the process of resonant Auger destruction (RAD), which occurs when a photon emitted by an ion is absorbed in a neighboring cooler part of the stellar wind by near-coincident inner-shell transitions of lower charge state ions. The inner-shell excited ion then undergoes Auger decay, in which the energy is transferred to an outer electron that is subsequently ejected from the atom by autoionization. EBIT measurements at a synchrotron beamline determined that 3d – 2p transitions of the lower iron charge state Fe VI is nearly coincident in transition energy with the Fe XVII 3G line, which would enable possible destruction of Fe XVII 3G photons and thus a potential explanation of the lower line intensity ratio found in ζ Pup. Model calculations show a noticeable amount of optical thickness for the Fe VI line, but the calculated X-ray line profile model does not show nearly enough reduction of the Fe XVII 3G line to suggest that RAD by Fe VI lines is causing the ratio anomaly in ζ Pup. Finally, I introduce preliminary steps for the analysis of XRISM spectral observations of Fe Kα lines from the starburst galaxy Messier 82. The key unsolved questions regarding M82 are what drives the hot wind and how much gas escapes the galaxy. Understanding the hot wind requires accurate measurements of its energy content, which requires obtaining constraints for the density, temperature, and velocity at the wind’s base. In order to sufficiently constrain the hot component velocity, the 6.7 keV Fe XXV line width and center must be determined to better than 10%. This accuracy requires an energy resolution ΔE ≤ 5 eV, which can be achieved by the high-resolution X-ray measurements with the XRISM Resolve calorimeter array. The M82 observation and subsequent analysis will confirm whether hot gas pressure is the primary driver of the galactic wind by measuring the energy contained in the T ∼ 10^8 K hot gas, and will constrain the mass-loading rate by measuring the velocity of the superheated nuclear gas using the Fe XXV line width. By completing these works, I will have successfully contributed to the refinement and advancement of theoretical, laboratory, and observational X-ray astrophysical data for iron transition lines.Item CONNECTING THEORY AND OBSERVATIONS OF EXOPLANET ATMOSPHERES AND SURFACES AT THE INDIVIDUAL AND POPULATION LEVEL WITH JWST(2024) Ih, Jegug; Kempton, Eliza M.-R.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Observing an exoplanet’s atmosphere via photometry and spectroscopy has provided the main window to understanding its properties and processes, as the atmospheric spectra encompass information about the chemistry, thermal structure, surfaces, as well as formation history and even biology. To this end, one key science goal of the James Webb Space Telescope (JWST) is to establish whether rocky planets around M dwarfs can host atmospheres or not. JWST offers unprecedented signal-to-noise and unlocks new parameter space regimes of planets available for characterizing not only the atmosphere but also the surface. This advancement in observing capability simultaneously poses novel challenges to atmospheric characterization. My dissertation addresses some of the new challenges to atmospheric retrievals in the era of JWST and the characterization of rocky planets. Firstly, I quantified the effects of wavelength-correlated systematics on atmospheric retrievals. Wavelength-correlated noise can occur due to instrumental systematics or stellar effects and the merging of discrete data sets. I investigated the effect of correlated noise and constrained the potential biases incurred in the retrieved posteriors by performing retrievals on multiple noise instances of synthetic data. The study found that correlated noise allows for overfitting the spectrum, thereby yielding a better goodness of fit on average but degrading the overall accuracy of retrievals by roughly 1σ. In particular, correlated noise can manifest as an apparent non-Rayleigh slope in the optical range, leading to an incorrect estimate of cloud/haze parameters. Finally, I show that while correlated noise cannot be reliably distinguished with Hubble Space Telescope observations, inferring its presence and strength may be possible with JWST. Secondly, I studied the how the choice in parameterization of the atmospheric composition can influence the posterior when performing retrieval analyses on terrestrial planet atmospheres, for which the mean molecular weight is not known a priori. By performing self-retrievals and varying the parameterization, I found that the centered log-ratio transform, commonly used for this application, tends to overestimate the abundances of spectroscopically active gases when inactive ones are present. Over multiple noise instances, I found that no one parameterization method always outperforms others. Comparing the Bayesian evidences from retrievals on multiple noise instances, I found that for a given spectrum, the choice in parameterization can affect the Bayes factor of whether a molecule should be included. Alongside astrophysical effects, this remains a fundamental challenge to atmospheric retrievals for small planet and can addressed by more observations. Finally, I constrained the atmospheric thickness and characterized the surface from the first JWST measurement of thermal emission from a rocky exoplanet, TRAPPIST-1 b. I compared TRAPPIST-1 b’s measured secondary eclipse depth to predictions from a suite of self-consistent radiative-convective equilibrium models. I found that plausible atmospheres (i.e., those that contain at least 100 ppm CO2) with surface pressures greater than 0.3 bar are ruled out at 3σ, regardless of the choice of background atmosphere, and a Mars-like thin atmosphere with surface pressure 6.5 mbar composed entirely of CO2 is also ruled out at 3σ. I modelled the emission spectra for bare-rock planets of various compositions and found that a basaltic surface best matches the measured eclipse depth to within 1σ.Item The Lives and Times of Stars and Black Holes in the Disks of Active Galactic Nuclei(2024) Dittmann, Alexander Joseph; Miller, Michael C; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Enormous disks of gas are thought to feed the supermassive black holes at the centers of active galaxies; these disks may capture stars from nuclear clusters, or form stars in situ after collapsing under their own gravity. Such stellar populations may enrich these accretion disks with fusion byproducts, cause giant flares in these active galaxies, and leave behind compact remnants detected on earth through gravitational waves emitted as they merge with one another. This dissertation charts a theoretical expedition into these phenomena, from studying the implications of star-forming accretion disks for the growth of black holes in the early universe, to simulating the flow of gas around black hole binaries to ascertain their orbital evolution. After a brief observational and theoretical overview of stars and active galactic nuclei, this dissertation delves into the development of simplified models of accretion disk structure, including the effects of stars and black holes embedded within accretion disks. The ultimate goal of this chapter was to determine if gravitational instability in the outer regions of these accretion disks might lead to the formation of large numbers of black holes, which might go on to merge with the central supermassive black hole; this process might decrease the effective radiative efficiency of accretion onto supermassive black holes, facilitating the rapid growth of black holes in the early universe, which defies conventional explanation. Along the way, this work developed a new flavor of model to describe these disks, accounting for the pressure support provided by feedback from disk-embedded stellar-mass black holes, developed a number of semi-analytical estimates for how stars might evolve within these accretion disks, and estimated the typical timescales for objects to move through the disk. Together, these estimates showed that accelerated supermassive growth in the early universe was indeed feasible, although this estimate hinged on a number of yet-untested assumptions. Subsequently, this dissertation advances to the question of how stars evolve when embedded within hot, dense disks of gas accreting onto supermassive black holes. Moving beyond the semi-analytical models of the preceding section, the third chapter reviews simulations of stellar evolution subject to the extreme conditions within these accretion disks. Stellar evolution calculations, due to the enormous spatial and time-scales involved, are virtually always restricted to one spatial dimension. This chapter investigates a number of the ways to account for the deviations in spherical symmetry inherent to accretion disks in these calculations, before reviewing how stellar rotation and the chemical composition of these accretion disks can affect the evolution of stars embedded therein. This work developed analytical criteria governing different regimes in stellar evolution, such as the balance between the stellar accretion and nuclear burning timescales, the relationship between gas composition and gas opacity, and the limiting effect of the central supermassive black hole's gravity on stellar accretion as the two compete for gravitational influence on the gas within the disk. Ultimately, the precise, quantitative details of these simulations depend on the specific 3D-inspired prescriptions implemented, but the overall trends identified are robust. The final study presented in this dissertation investigates the feasibility of these accretion disks as the host sites for the stellar-mass black hole mergers detected by the Laser Interferometer Gravitational-Wave Observatory. One of the primary uncertainties of this scenario is whether binaries formed within the disk will tend to spiral inward after formation, or instead be driven via hydrodynamic interactions to spiral outward to the point where chaotic three-body interactions would separate the binary. To address the feasibility of this gravitational wave progenation channel, we conducted three-dimensional hydrodynamical simulations of black hole binaries embedded within these accretion disks, at orbital separations slightly smaller than the limit for dynamical instability. This chapter focused on initially circular binaries over a range of orbital inclinations with respect to the midplane of the disk, finding that binaries with orbits at all misaligned with the disk midplane are gradually realigned, and that retrograde binaries can inspiral appreciably faster than prograde ones. Although the simulations were physically incomplete, in particular neglecting magnetohydrodynamic and radiative effects, they suggest that AGN disks could indeed host the binary black hole mergers detected via gravitational waves.Item Interactions between Massive Stellar Feedback and Interstellar Gas in the Eagle Nebula(2024) Karim, Ramsey Lee; Mundy, Lee G; Pound, Marc W; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)My thesis describes multi-scale stellar feedback processes observed in the Eagle Nebula star forming region in our Milky Way galaxy. Stellar feedback from massive stars encompasses bright ultraviolet radiation which ionizes atoms and dissociates molecules in gas surrounding the stars as well as supersonic winds which impact the gas and create hot shocked layers. I study the interaction of stellar radiative and mechanical feedback with pre-existing density inhomogeneities in the molecular cloud in order to learn about the effects of the interstellar environment on the relative efficiency of various forms of feedback. This work informs our understanding of the life cycle of interstellar gas: gas forms stars and is then exposed to their winds and radiation, and we would like to know how that affects the formation of future generations of stars. The Eagle Nebula's relative proximity to us means we observe the H II region with high spatial resolution. Extra-galactic studies observe many H II regions simultaneously and at a variety of cosmic ages, but lack the resolution to understand the structure of the individual regions. High resolution studies of Galactic sources such as the Eagle serve as templates for what extra-galactic astronomers are seeing in far-away galaxies. The work also contributes to sub-grid feedback prescriptions in large-scale simulations of galaxy formation and evolution. Stars and their feedback are too small to be simulated in these contexts, so theorists require accurate approximations for the effects of stellar feedback. Massive stars form in massive molecular gas clouds and then deliver vast quantities of energy back into the clouds in the form of radiation and stellar winds. They form H II regions, 1-to-10-light-year scale areas of ionized hydrogen, which are often overpressured bubbles compared to the surrounding interstellar medium, and their supersonic winds sweep up a compressed shell of gas. Around the edge of the H II region, there lies a layer of gas which receives no >13.6 eV extreme-ultraviolet H-ionizing radiation (EUV), but is rich in 6-13.6 eV far-ultraviolet radiation (FUV) which can photodissociate molecules such as CO and H2 and ionize C. These photodissociation regions (PDRs) are heated via the photoelectric effect as FUV radiation interacts with organic molecules called polycyclic aromatic hydrocarbons (PAHs), and the regions are cooled by the collisionally excited far-infrared fine structure transitions of ionized carbon and atomic oxygen. The FEEDBACK SOFIA C+ Legacy Project (Schneider et al. 2020) studies the coupling efficiency of that energetic feedback to the gas by observing one such transition of singly ionized carbon at 158 micron referred to as C+ or [C II]. In this astrophysical context, the line is emitted primarily within PDRs. With modern heterodyne receivers and an observatory above Earth's atmosphere, we can both detect and spectroscopically resolve the [C II] line and therefore trace the morphology and kinematics of the PDR regions surrounding massive stars. We contextualize these observations with velocity-resolved observations tracing the un-illuminated molecular gas beyond the PDRs and a variety of archival data spanning the electromagnetic spectrum from radio to X-ray. I use these observations to study the Eagle Nebula, home to the iconic Pillars of Creation, and learn how pre-existing density structure evolves when exposed to stellar feedback and what that implies for the energetic coupling of the stellar feedback to the gas. My first study covers the Pillars of Creation in a detailed, multi-wavelength analysis published in the Astronomical Journal. We find that these pillars are long-lasting structures on the scale of the H II region age and that they must arise from pre-existing density structures. My second study zooms out to the greater Eagle Nebula H II region to learn how the massive stars affect the rest of the region. This analysis concludes that the primordial filamentary structure which must have led to the formation of the stellar cluster also governs the shape of the H II region and how much of the surrounding gas is affected by the feedback. Finally, I describe a software package, scoby, which I developed to aid these two studies. The software connects theoretical feedback estimates to observed star catalogs and delivers results tuned for observational studies like these. It has been used for several published analyses of other regions.Item Population Studies of Tidal Disruption Events and Their Hosts: Understanding Host Galaxy Preferences and the Origin of the Ultraviolet and Optical Emission(2024) Hammerstein, Erica; Veilleux, Sylvain; Cenko, S. Bradley; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)It is well-established that the majority of galaxies harbor a supermassive black hole (SMBH) in their nucleus. While some of these SMBHs are easily studied either through signatures of persistent gas-fueled accretion or direct observations of the SMBH's influence on stars and gas in its potential well, many more are elusive, providing no obvious evidence of their existence. One way to detect these dormant SMBHs is through the tidal disruption of a star that wanders too close and is torn apart under the tidal stress. These tidal disruption events (TDEs) illuminate otherwise difficult-to-study dim or distant galaxy nuclei, acting as cosmic signposts announcing the presence of the SMBH lurking there through luminous flares observed across the electromagnetic spectrum. These flares can, in principle, be used to extract information about the SMBH itself, and can therefore serve as important probes of SMBH growth and evolution. TDE host galaxies can be used to study the connection between SMBHs and their environments, an important goal in understanding the origin of SMBHs, galaxy formation, and SMBH co-evolution. My dissertation addresses both of these important facets of TDEs, their light curves and their hosts, to understand not only the events themselves but how they can be used to study SMBHs. First, I studied a sample of 30 optically selected TDEs from the Zwicky Transient Facility (ZTF), the largest sample of TDEs discovered from a single survey yet. After performing a careful light curve analysis, I uncovered several correlations between light curve parameters which indicate that the properties of the black hole are imprinted on the light curve. I also fit the light curves using tools that yield black hole mass estimates and I found no correlation between these estimates and the host galaxy stellar mass. I found no difference between the optical light curve properties, apart from the peak luminosity, of the X-ray bright and X-ray faint TDEs in this sample. This provides clues as to the origin of the optical emission and may support a scenario where the viewing angle is responsible for the observed emission. Lastly, I presented a new spectral class of TDE, TDE-featureless, which in contrast to other events, show no broad lines in their optical spectra. This new class may be connected to the rare class of jetted TDEs. Next, I studied a subset of host galaxies in the ZTF sample of TDEs. I examined their optical colors, morphology, and star-formation histories. I found that TDE hosts can be classified as ``green'', in a phase between red, inactive galaxies and blue, star-forming galaxies. Morphologically, the TDE hosts are centrally concentrated, more so than galaxies of similar mass and color. By looking at the optical spectra of the TDE hosts, which can be used to estimate the current star formation and the star formation history, I found that TDE host populations are dominated by the rare class of E+A, or post-starburst, galaxies. In tandem with the other peculiar photometric and morphological properties, this points to mergers as the likely origin for TDE hosts. I extended this study of TDE hosts by using integral field spectroscopy to infer black hole masses via the $M_{\rm BH} - \sigma_\star$ relation and investigate large-scale stellar kinematics. I found that the black hole mass distribution for TDE hosts is consistent with the theoretical prediction that they should be dominated by lower mass SBMHs. Interestingly, one TDE-featureless object was found to have a black hole mass of $\log(M_{\rm BH}/M_\odot) = 8.01$, which is likely above the Hills mass for the disruption of a solar-type star and could necessitate a rapid spin for this particular black hole. If high spin is required to launch relativistic jets, this may further support the connection between featureless TDEs and jetted TDEs. The large-scale kinematics of a galaxy are strongly tied to its merger and star formation history. I found that TDE hosts share similar kinematic properties to E+A galaxies, which are thought to be post-merger. Lastly, I presented further observations of the jetted TDE AT2022cmc. This event, discovered in the optical, presented an opportunity to place this rare class of TDE in the context of the larger TDE population. I performed a careful light curve analysis that accounts for both the thermal and non-thermal components in the light curve. I showed that the thermal component of AT2022cmc is similar to the TDE-featureless class of events and follows correlations presented for TDE light curve properties found in this thesis.Item THE FORMATION OF METAL-FREE POPULATION III STARS IN X-RAY AND LYMAN-WERNER RADIATION BACKGROUNDS(2024) Park, Jongwon; Ricotti, Massimo; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Metal-free Population III (or Pop III) stars are instrumental in shaping the early universe, influencing the formation of the first galaxies. The formation of Pop III stars depends on the fraction of molecular hydrogen (H2), which is regulated by X-ray and Lyman-Werner (LW) radiation backgrounds. Therefore, gaining insight into the impact of these radiation backgrounds is essential for unraveling the mysteries surrounding Pop III star formation and their impacts on the first galaxies. In this dissertation, I investigate the interaction between X-ray/LW backgrounds and the formation of Pop III stars. To conduct this investigation, I employed the radiative hydrodynamics code RAMSES - RT. I implemented various physical processes governing Pop III star formation, such as primordial chemistry, radiation background, secondary ionization/heating, and self-shielding. Performing a grid of simulations covering a large parameter space of X-ray/LW intensity, I systematically explored the effects of radiation backgrounds on Pop III stars. I found that a moderate X-ray background boosts the H2 fraction in dark matter halos, facilitating Pop III star formation in low-mass halos. In contrast, a LW background dissociates H2 and prevents star formation in low-mass halos. This result suggests that the number of Pop III supernovae detected by the JWST is enhanced by an X-ray background. Furthermore, I discovered that an X-ray background reduces the characteristic mass and multiplicity of Pop III stars. This leads to a top-heavier initial mass function and may have a potential impact on galaxy formation. Moreover, I made further improvements to the simulations by incorporating radiative feedback from Pop III protostars. This study confirmed previous works that radiation from protostars suppresses their growth, thereby playing a significant role in determining the mass of Pop III stars theoretically. I also found that hierarchical binaries (binaries of binaries), eccentric orbits, and outward migration are common occurrences in Pop III star formation. Eccentric orbits induce variability of Pop III protostars and this is observable by the JWST when light is magnified through gravitational lensing. In a follow-up study, I investigated the origin of outward migration and found that the gas disks around the protostars accrete gas with high angular momentum and transfer the angular momentum to the binary stars through torques. This finding paves the wayfor studies of migration behaviors across different stellar populations. Finally, I explored the X-ray effects on the number of Pop III stars using cosmological simulations. Developing methods to calculate the intensity of the radiation background on the fly and realistically accounting for the X-ray feedback loop, I found that a weak X-ray background develops and this background ionizes the intergalactic medium, thereby moderately increasing the number density of Pop III stars (by a factor of ∼ 2). This rise in the number of Pop III stars due to X-ray radiation lowers the star formation rate of metal-enriched Pop II stars, highlighting the significance of the X-ray background in galaxy formation.This thesis covers various aspects of Pop III star formation and the effect of X-ray radiation backgrounds which has been overlooked by previous studies. It lays a foundation for future research aimed at connecting the theoretical understanding of Pop III star formation and observations targeting Pop III stars and the first galaxies.Item Mechanical evolution of small solar system bodies(2023) Marohnic, Julian Charles; Richardson, Derek C; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation presents a series of studies that develop and apply numerical modeling techniques to small bodies in the solar system. We are particularly interested in low-energy deformations, collisions, and disruptions, and our subjectsrange from near-Earth asteroids to Kuiper belt contact binaries in the farthest reaches of the solar system. We use the N-body code pkdgrav to investigate these processes and describe our significant additions to its capabilities. Our first subject is the Kuiper belt object Arrokoth. On January 1, 2019, the New Horizons spacecraft flew within 3,550 km of Arrokoth, returning the first in-situ images of a small body in the classical Kuiper belt. Arrokoth was found to be bilobate, with a distinctive contact binary structure. We use pkdgrav to investigate the origins of Arrokoth's striking shape and find that plausible formation mechanisms are quite limited. We rule out the possibility of a direct impact between two unbound objects and put forward an alternate scenario in which two cometesimals in a close, synchronous orbit gradually spiral in toward one another before meeting in a gentle merger. We conclude by exploring implications for the formation of small Kuiper belt objects more generally. Next, we describe our work modifying pkdgrav to accommodate non-spherical particles. Prior work in granular physics has established that particle shape is an important factor governing the behavior of granular bodies like small solar system objects. Irregular particles tend to interlock with one another, inhibiting bulk motion and adding to the shear strength of a medium. We adapt pkdgrav's existing soft-sphere, discrete element contact physics model to allow for modeling of non-spherical grains. We then apply this new capability in three, small-scale proof of concept studies of spin-up, tidal disruption, and the Brazil nut effect. We find a significant difference in behavior when comparing small rubble-pile bodies composed of spherical particles and those composed of non-spherical particles. Finally, we apply our newly-developed tools to a more comprehensive investigation of particle shape in tidal disruption simulations. We construct small rubble piles from a range of differently-shaped constituents and subject them to simulated tidal encounters with the Earth. We conduct a parameter sweep across different encounter geometries and constituent shapes and conclude that particle shape is a significant contributor to tidal encounter outcomes. The role of particle resolution is also investigated.Item The Shadows of Would-Be Gods: Finding Transiting Jovians, Terrestrials, and Everything in Between with TESS to Understand Hot Jupiter Formation and the Best Targets for JWST(2023) Hord, Benjamin James; Kempton, Eliza; Colón, Knicole; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)NASA’s Transiting Exoplanet Survey Satellite (TESS) mission launched in 2018 and has since observed more than 90% of the sky and discovered more than 6,000 planet candidates of many sizes, temperatures, and orbital periods. Hot Jupiters, in particular, have benefited from TESS since these planets are uniformly distributed throughout the sky and produce large transit signals. Many questions remain about this enigmatic class of large gas giants orbiting extremely close to their host stars regarding their formation and evolution. My dissertation leverages TESS to investigate the potential formation mechanisms of hot Jupiters and applies relevant planet discovery techniques to a collection of planet candidates that would be most amenable, or “best-in-class,” for atmospheric characterization with JWST. First, I performed a uniform search for nearby companion planets to hot Jupiters observed by TESS in its first year of operations. The lack of planets nearby hot Jupiters in their planetary systems has long been thought to be a fingerprint of their dynamically active formation history, although a recent set of discoveries of nearby planets in three hot Jupiter systems has challenged this notion. I developed a custom-built search, vetting, and validation pipeline to detect additional transit signals in TESS light curves of hot Jupiter systems and evaluate the planetary nature of each. This study found a host of new transit-like signals but none were deemed to be caused by planets, reinforcing the idea that companion planets to hot Jupiters are rare. I also estimated the expected rate at which hot Jupiters should have companions and found it to be 7.3+15.2−7.3%. Second, I continued the search for additional planets in hot Jupiter systems as TESS continued to observe the sky and discovered a new signal in the WASP-132 system. I vetted and statistically validated this signal to demonstrate that it is indeed from a new planet, dubbed WASP-132c. This planet orbits interior to the hot Jupiter WASP-132 b and constitutes only the fourth such system discovered at the time. I performed some initial analysis on the limited sample of hot Jupiters with nearby companions and found evidence suggesting that systems with this architecture predominantly have an outer hot Jupiter beyond the ∼3 day orbital period pileup with an inner companion. This may be due to a number of factors, including physical and observational, such as formation mechanism or the bias towards short period planets of transit surveys. Finally, I leveraged the planet discovery, vetting, and validation techniques I had applied to the search for companions to hot Jupiters to perform a large-scale validation of over 100 planet candidates discovered by TESS that were deemed “best-in-class” for atmospheric characterization with JWST. This included the synthesis and ranking of all planets and planet candidates by observability with JWST into a single sample and then performing vetting and validation analyses on those that were candidates. In total, I statistically validated 22 planet candidates and marginally validated a further 35. I present the final best-in-class sample as a community resource for future JWST observations.Item Investigating the X-ray temporal and spectral properties of blazars and beamed AGN in the Swift-BAT Hard X-ray Survey(2023) Mundo, Sergio A.; Mushotzky, Richard; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Blazars are generally known to exhibit high-amplitude, rapid variations in flux, polarization, and in their spectra across most timescales and wavelengths. While the consensus for these objects is that their emission is indeed ``highly variable", a more specific characterization of the variability may depend on the timescales considered. In this dissertation, I investigate the nature of the variability of these objects and the physical processes involved in producing it, through the lens of blazars that have been detected by the Swift Burst Alert Telescope. My foray into the high-energy astrophysics of blazars begins with a case study of a blazar-like AGN. For the first time for this source, I definitively measure X-ray reflection features and help determine the origin of its broadband X-ray emission, suggesting that the X-rays from this object predominantly come from regions in the vicinity of the black hole, while also finding evidence of jetted emission in the hard X-rays. I further explore blazar X-ray emission by investigating the rest of the blazars in the Swift-BAT survey, and in doing so I conduct the first study in the time domain dedicated to the hard X-ray variability behavior of blazars on long timescales based on ~13 years of continuous X-ray data in the 14-195 keV band. In this study, I find that a significant portion of the blazars in the sample (~37%) do not show statistically significant variability on monthly timescales, which is in tension with the expected high variability of blazars seen in previous studies. In addition, I show that for some of the brightest blazars, the long-term spectra in the hard X-rays may be described in a relatively simple way, with a power law that changes slope on monthly timescales. Since the BAT data are not sensitive to changes on shorter timescales, or to low-amplitude variability on monthly timescales, I follow up on the supposedly ``non-variable" blazars from the previous investigation by using recent NICER observations of a sub-sample of 4 such “quiescent” BAT blazars over 5 months, allowing for insight into the short-timescale and lower amplitude variability while also representing some of the longer timescales sampled by the BAT survey. I show that variations in the NICER band are in fact detected on several timescales, but that the fractional variability appears to decrease with longer timescales, implying generally low-amplitude variability across all sources and showing very low variability on monthly timescales, which is once again at odds with studies that have shown that blazars are highly variable in the X-rays on a wide range of timescales. I also show through a spectral analysis that the broadband X-ray spectra (0.3-195 keV) of these sources can be described with different power law models, with one source requiring significant absorption in the soft X-rays to fully describe its observed curvature, possibly due to absorption in the intergalactic medium. Additional observations from a new follow-up NICER campaign will further facilitate probing the variability of these BAT blazars for up to timescales of a year, serving as an additional stepping stone towards our ultimate goal of characterizing the X-ray variability of blazars and beamed AGN.Item A Song of Fire & Ice: Evolutionary Properties of Hot & Cold Small Bodies(2023) Holt, Carrie; Knight, Matthew M.; Richardson, Derek C.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Small bodies (i.e., asteroids and comets) play an important role in our understanding of the Solar System. They are composed of the same planetesimal material that was incorporated into the planets, but their smaller size kept them from experiencing extensive processing (such as differentiation or atmosphere-related surface erosion). Therefore, their primitive nature allows us to probe the composition of the early Solar System and its subsequent evolution. Even though comets and asteroids are expected to contain material characteristic of their formation region, they have undoubtedly undergone some degree of processing since they were formed. The overarching motivation for small-body science is to disentangle primordial characteristics from evolutionary characteristics developed since formation with the goal of better understanding how our Solar System came to be. This work seeks to tackle a small piece of this goal by studying the objects of two extreme populations: the most and least thermally processed bodies. This thesis uses ground-based broadband optical photometry to investigate the differences between different small body populations and how thermal processing alters the characteristics of objects over time. First, we investigate the optical colors of near-Sun asteroids that experience extreme temperatures of > 1000 K to better understand the dominant processes that affect their surface properties and could potentially lead to their disruption. Next, we characterize the long-term brightness evolution of long-period comets using two distinct datasets: 1) an observing campaign that conducts long-term monitoring of long-period comets that are active beyond the region where water-ice sublimation is efficient, and 2) photometric magnitudes of long-period comets with well-characterized orbits that were collected and reported by amateur observers. We assess our ability to improve brightness predictions for comets discovered at large heliocentric distances and establish if brightness behavior can be used as a diagnostic of dynamical age.