Astronomy Theses and Dissertations

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    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.
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    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.
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    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.
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    New Messengers & New Physics: A Survey of the High-energy Universe
    (2023) Crnogorcevic, Milena; Ricotti, Massimo; Caputo, Regina; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Studying the origins of the high-energy emission in the Universe can profoundly affect our fundamental understanding of the cosmic origin and its evolution at the most extreme scales. In this dissertation, I explore the high-energy observations of different astrophysical systems to inform our understanding of the theoretical frameworks used to describe them. I harness the current multimessenger infrastructure to investigate questions ranging from new physics and transient astronomy to compact objects and extended emission in the gamma-ray, gravitational-wave, and neutrino skies. The focus in the first part of this dissertation is on utilizing the Fermi Large Area Telescope (LAT) low-energy (LLE) technique to search for the light axion-like-particle (ALP) within the MeV gamma-ray emission of long gamma-ray bursts (GRBs). We perform a data-driven sensitivity analysis to determine distances for which detection of an ALP signal is possible with the LLE technique, which, in contrast to the standard LAT analysis, allows for a larger effective area for energies down to 30 MeV. Assuming an ALP mass $m_a \lesssim 10^{-10}$~eV and ALP-photon coupling $g_{a\gamma} = 5.3\times 10^{-12}$ GeV$^{-1}$, we find that the distance limit ranges from $\sim\!0.5$ to $\sim\!10$~Mpc. We demonstrate that the sensitivity of the LLE technique to detecting light ALPs is comparable to the standard LAT analysis, making it an excellent complementary---yet independent---way to search for ALPs with \textit{Fermi}. Next, we select a candidate sample of twenty-four GRBs and conduct a model comparison analysis in which we consider different GRB spectral models with and without an ALP signal component. We find that including an ALP contribution does not result in any statistically significant improvement of the fits to the data. Motivated by the delay between the ALP emission time and the time of the jet break-out associated with its ordinary long-GRB emission, we conduct a novel search for ALPs within time windows that precede the main-episode gamma-ray emission of a long GRB, focusing on the sample of sources with known precursor emission detected with LAT and LLE. We report no statistically significant detection of ALPs within the GRB precursor emission and discuss the parts of the ALP parameter space probed with this method. Multimessenger astronomy is at the heart of the remainder of this dissertation. First, I present a follow-up search for excess emission of X-rays with the Swift Burst Alert Telescope (Swift-BAT) and that of gamma rays with the Fermi Gamma-ray Burst Monitor (Fermi-GBM), in spatial and temporal correspondence to gravitational-wave events reported by the LIGO/Virgo/Kagra (LVK) Collaboration. In collaboration with the Fermi-GBM Team, we combine the observations from these two instruments to determine whether there is any statistically significant excess emission around the given gravitational-wave trigger. We report no new joint detections but present the joint flux upper limits. Finally, I present the results of the cross-correlation studies between the unresolved Fermi-LAT gamma-ray and the IceCube neutrino skies. We report no positive cross-correlation in the real-data sky maps. We then combine simulation and observation techniques to place upper limits on the fraction of neutrinos produced in proton-proton or proton-gamma interactions that occur in blazars. Assuming all gamma rays from unresolved blazars are produced from neutral pions via proton-proton interactions, we find that---for energies above 10~GeV---up to 60 % of the unresolved blazar population may contribute to the diffuse neutrino background (the fraction is 30 % for proton-gamma interactions). We also include predictions for the improved sensitivity considering 20 years of IceCube data.
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    Simulating Bursty and Continuous Reionization Using GPU Computing
    (2023) Hartley, Blake Teixeira; Ricotti, Massimo; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Reionization is the process by which the neutral intergallactic medium of the early universe was ionized by the first galaxies, and took place somewhere between roughly redshift 30 and redshift 6, or from 100~Myr into the universe to 1~Gyr. The details of this transition are still not well understood, but observational constraints suggest that reionization happened faster than naive estimates would suggest. In this thesis, we investigate the theory that galaxies which form their stars in short bursts could complete reionization faster than galaxies which emit their photons continuously over their lifespans. We began investigating this theory with a semi-analytic model of the early universe. We used analytic methods to model the expansion of \HII (ionized hydrogen) regions around isolated galaxies, as well as the behavior of the remnant \HII regions after star formation ceases. We then compiled assortments of galaxies matching dark matter simulation profiles and associated each with an \HII region that could either grow continuously or grow quickly before entering a dormant period of recombination. These tests indicated that the remnants of bursty star formation had lower overall recombination rates than those of continuously expanding \HII regions, and that these remnants could allow for ionizing radiation from more distant sources to influence ionization earlier. We decided that the next step towards demonstrating the differences between continuous and bursty star formation would require the use of a more accurate model of the early universe. We chose a photon conserving ray tracing algorithm which follows the path of millions of rays from each galaxy and calculates the ionization rate at every point in a uniform 3D grid. The massive amount of computation required for such an algorithm led us to choose MPI as the framework for building our simulation. MPI allowed us to break the grid into 8 sub-volumes, each of which could be assigned to a node on a supercomputer. We then used CUDA to track the millions of rays, with each of the thousands of CUDA cores handling a single ray. Creating my own simulation library would afford us complete control over the distribution and time dependence of ionizing radiation emission, which is critical to isolating the effect of bursty star formation on reionization. Once we had completed, we conducted a suite of simulations across a selection of model parameters using this library. Every set of model parameters we selected corresponds to two models, one continuous and one bursty. This selection allowed us to isolate the effect of bursty star formation on the results of the simulations. We found that the effects we hoped to see were present in our simulations, and obtained simple estimates of the size of these effects.
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    (2023) He, ChongChong; Ricotti, Massimo; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Star formation is a crucial process that lies at the center of many important topics in astrophysics: the nature of the first sources of radiation, the formation and evolution of galaxies, the synthesis of elements, and the formation of planets and life. Recent advances in computing technology have brought about unprecedented opportunities to deepen our understanding of this complex process. In this dissertation, I investigate the physics of star formation in galaxies and its role in shaping the galaxies and the Universe through numerical simulations.My exploration of star formation begins with a large set of simulations of star cluster formation from isolated turbulent Giant Molecular Clouds (GMCs) with stellar feedback using \ramses{}, a state-of-the-art radiation-magneto-hydrodynamic (radiation-MHD) code. While resolving the formation of individual stars, I have pushed the parameters (mass and density) of the simulated GMCs well beyond the limit explored in the literature. I establish physically motivated scaling relationships for the timescale and efficiency of star formation regulated by photoionization feedback. I show that this type of stellar feedback is efficient at dispersing dense molecular clouds before the onset of supernova explosions. I show that star formation in GMCs can be understood as a purely stochastic process, where instantaneous star formation follows a universal mass probability distribution, providing a definitive answer to the open question of the chronological order of low- and high-mass star formation. In a companion project, I publish the first study of the escape of ionizing photons from resolved stars in molecular clouds into the intercloud gas. I conclude that the sources of photons responsible for the epoch of reionization, one of the most important yet poorly understood stages in cosmic evolution, must have been very compact star clusters, or globular cluster progenitors, forming in dense environments different from today's galaxies. In follow-up work, I use a novel zoom-in adaptive-mesh-refinement method to simulate the formation and fragmentation of prestellar cores and resolve from GMC scales to circumstellar disk scales, achieving an unprecedented dynamic range of 18 orders of magnitude in volume in a set of radiation-MHD simulations. I show that massive stars form from the filamentary collapse of dense cores and grow to several times the core mass due to accretion from larger scales via circumstellar disks. This suggests a competitive accretion scenario of high-mass star formation, a problem that is not well understood. We find that large Keplerian disks can form in magnetically critical cores, suggesting that magnetic braking fails to prevent the formation of rotationally-supported disks, even in cores with mass-to-flux ratios close to critical. This is because the magnetic field is extremely turbulent and incoherent, reducing the effect of magnetic braking by roughly one order of magnitude compared to the perfectly aligned and coherent case, which proposes a solution to the ``magnetic braking catastrophe.''
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    Photochemistry of Exoplanet Atmospheres: Modelling alien chemistry accurately and self-consistently
    (2023) Teal, Dillon James; Kempton, Eliza; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Exoplanets offer unique physical and chemical laboratories experiencing entirely alien environments compared to the Solar System planets. Their atmospheres, governed by the same laws of physics, display remarkable diversity and complexity. They serve as the most complex planetary phenomena we can directly observe, coupled to the planet's interior processes, formation environment, the properties of the host star, and complex chemical ecosystems. The art of modelling these systems is a rich field of study, and in this work I study the nature of photochemical models and what understanding they can provide for us based on the quality and breadth of their inputs. By characterizing the implicit uncertainty chemical models have without a well-characterized host star, I quantify the importance of host star characterization to chemical modelling, showing their sensitivity under different reaction schemes and microphysical models. I then apply this to recent observations of known exoplanet host stars LHS 3844 and AU Microscopii. Finally, I cover work to model sub-Neptune atmospheres across a wide parameter space aimed at understanding the influence of a planet's environment and unknowns on haze formation and observational prevalence in emission and transmission spectroscopy.
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    (2023) Villanueva, Vicente; Bolatto, Alberto D; Vogel, Stuart N; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Star formation activity plays a key role in driving galaxy evolution, and it depends on the amount of cold gas available (as traced by CO emission) and on the physical conditions and dynamical state of this gas. This work focuses on investigating the star formation efficiency of the gas, both molecular and total, as a function of local and global galaxy properties. The galaxy samples studied in this thesis are representative of the nearby universe, and we use a combination of interferometric CO observations and integral field unit optical spectroscopy for most of our analyses. First, we show that in a sample of galaxies dominated by ``field galaxies'' the disk scale lengths for the molecular and stellar components are very similar, reflecting the close relation between CO emission and star formation activity. Our analysis of the radial dependence of the star formation efficiency of the total gas on morphological, structural, and dynamical properties of the galaxies shows that there is a smooth, continuous exponential decline with increasing radius (mostly driven by the increased contribution of atomic gas), and a systematic increase in the average gas efficiency from early to late type galaxies. Our results also show a morphological dependence of the efficiency per orbital time, which may reflect star formation quenching due to the presence of a bulge. Next, we test the impact of environmental processes on galaxies immersed in the rich environment of the Virgo cluster. We show that in these galaxies the CO emission is more centrally concentrated than the stars, unlike what we saw in the field. Moreover, in the central regions of galaxies with an increasing level of perturbation (as determined by the morphology and kinematics of their atomic gas emission), the mean molecular-to-atomic gas ratio increases while the star formation efficiency of the molecular gas in the same region decreases. This demonstrates that the cluster environment not only affects the outskirts of galaxy disks and their atomic gas, but deeply changes the distribution and efficiency of the centrally located molecular gas component. Finally, we study the onset of star formation cessation in galaxies (``quenching'') by investigating a complete sample of galaxies spanning from the main sequence (normal star forming objects) to the green valley (galaxies which are starting to quench) to the red cloud (galaxies that are mostly quiescent, that is, ``red and dead'' objects). We find that the star formation activity and the molecular gas-to-stellar mass ratio track each other. We also note that green valley galaxies have lower molecular star formation efficiencies than galaxies on the main sequence. On average, we find that within the bulges of green valley galaxies the molecular gas star formation efficiency is lower than in main sequence galaxies. Also in green valley galaxies, we find that the molecular gas to stellar ratio, the molecular gas star formation efficiency, and the specific star formation rate all increase with increasing distance to the center. Our results suggest that gas depletion or removal does not fully explain the star-formation quenching in galaxies transiting through the green valley, and that a reduction in star formation efficiency is also required during this stage. This is reminiscent of the so-called ``morphological quenching.''
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    (2023) Li, Jialu; Harris, Andrew; Tielens, Alexander; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Star formation processes originating from dense molecular clouds leave us a molecular universe. How molecules probe the physical conditions at different star-forming stages and how the physical environments control the formation of the chemical inventory becomes a key question to pursue. In the past, the understanding of this problem is impeded by instrumentallimitations. With instruments advanced in sensitivity and spatial/spectral resolution, this thesis investigates the molecular environment of different star-forming regions. Half of this thesis (Chapter 2 and Appendix A) focuses on mapping cold dense molecular gas in an external galaxy, IC 342, at 3 Mpc. The distribution of molecular gas was efficiently mapped with a set of density-sensitive tracers with Argus. Argus is the first array receiver functioning at 3 mm on the 100 m Green Bank Telescope (GBT) and provides a resolution of 6′′–10′′. As this study was conducted in the early era of Argus’ deployment, valuable information on the instrument’s behavior is learned. The resolved molecular maps characterize the fundamental physical properties of the clouds including the volume density and the excitation conditions. Comparisons with results from radiative transfer modeling with RADEX help to decrypt this information. The high spatial resolution of Argus also provides an opportunity in inspecting a scale-scatter breakdown of the gas density-star formation correlation in nearby galaxies and in investigating the influence of a finer spatial resolution on the correlation. The other half of the thesis (Chapters 3 and 4) studies the hot core, an embedded phase during massive star formation, of a proto-binary system W3 IRS 5 at 2.2 kpc. Rovibrational transitions of gaseous H2O, CO, and isotopologues of CO were detected with mid-IR absorption spectroscopy. The high spectral resolution (R ∼50,000–80,000) not only separates each transition individually but also decomposes different kinematic components residing in the system with a velocity resolution of a few km/s . Physical substructures such as the foreground cloud, high-speed “bullet”, and hot clumps in the disk surface are identified. Characterization of the physical substructures is conducted via the rotation diagram analysis and curve-of-growth analyses. The curve-of-growth analyses, under either a foreground slab model or a disk model, take account of the optical depth effects and correct the derived column densities by up to two orders of magnitude. The disk model specifically suggests a disk scenario with vertically-decreasing temperature from mid-plane, which is intrinsically different from externally illuminated disks in the low-mass protostellar systems that have hot surfaces. Connections between physicalsubstructures and chemical substructures were also established. Investigations on chemical abundances along the line of sight reveal the elemental carbon and oxygen depletion problem.
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    (2022) Tarantino, Elizabeth; Bolatto, Alberto D; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Gas and dust in the interstellar medium (ISM) cools and condenses, gravitationally collapses, and forms stars. At the same time, stars can heat and ionize their surroundings, influencing the physical conditions of the nearby ISM. In this thesis, I take a multi-wavelength, spectroscopic approach to investigate the physical conditions of the multi-phase ISM in nearby galaxies. The [CII] fine-structure transition at 158 micron is frequently the brightest far-infrared line in galaxies and can trace the ionized, atomic, and molecular phases of the ISM. I present velocity-resolved [CII] observations from SOFIA in the nearby galaxies M101 and NGC 6946 and determine that [CII] emission is associated with the atomic and molecular gas about equally, with little contribution from the ionized gas. Using the [CII] cooling function, I calculate the thermal pressure of the cold neutral medium and find that the high star formation rates in our sample can drive large thermal pressures, consistent with predictions from analytical theory. Next, I investigate the properties of the ionized gas around one of the hottest and most luminous Wolf-Rayet (WR) stars in the Small Magellanic Cloud. I use spatially resolved mid-infrared Spitzer and far-infrared Herschel spectroscopy to establish the physical conditions of the ionized gas. Using the photoionization code Cloudy, I construct models with a range of constant densities between n_H = 4 - 12 cm^-3 and a stellar wind-blown cavity of 15 pc that reproduce the intensity and spatial distribution of most ionized gas emission lines. The higher ionization lines cannot be produced by the models --- however, I show that wind-driven shocks or a harder ionizing WR spectrum can explain their intensities. Lastly, I explore the properties of molecular clouds in a large (170x350 pc) map of an active star-forming region in the Large Magellanic Cloud. Using 12CO(2-1) and 13CO(2-1) observations from the ALMA ACA, I decompose the emission into individual cloud structures and determine their sizes, linewidths, mass surface densities, and virial parameters. Almost all of the clouds are gravitationally bound or marginally bound and share similar properties to molecular clouds in the Milky Way. I do not find evidence that the surrounding star formation significantly influences the kinematic properties of the clouds through stellar feedback.
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    On the Dynamics of Binary Asteroids Applied to DART Mission Target (65803) Didymos
    (2022) Agrusa, Harrison Fitzgerald; Richardson, Derek C; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    NASA’s Double Asteroid Redirection Test (DART) mission will be the first full-scale demonstration of a kinetic impactor for planetary defense. On September 26, 2022, the DART spacecraft is expected to impact Dimorphos, the secondary component of the Didymos binary asteroid system. The DART impact will reduce Dimorphos’s relative orbital velocity, shrinking both its semimajor axis and orbit period. The mutual orbit period will then be measured us- ing ground- and space-based observations in order to deduce the momentum transfer efficiency, which is an important parameter in planetary defense that has never been measured experimentally at a realistic scale. This thesis comprises a set of studies on the spin and orbital dynamics of the Didymos system conducted in support of the DART mission. Owing to the close proximity of Didymos and Dimorphos and their irregular shapes, the mutual dynamics are non-Keplerian and exhibit a high degree of spin-orbit coupling, which often requires the use of specialized numerical methods to model the system. First, we conducted a benchmarking and sensitivity study to identify the best simulation codes for future DART-supported studies and to understand how small perturbations in the initial conditions can affect the resulting dynamical evolution of the system. Then, we demonstrated that Dimorphos can enter a wide range of post-impact spin states, including possible chaotic non-principal axis rotation, depending on its shape and the amount of momentum transferred by the DART impact. We then explored the implications of an excited spin state, including the possibility of ongoing granular motion on Dimorphos’s surface resulting from the orbital perturbation induced by the DART impact. This thesis is focused predominantly on the dynamics of the Didymos binary. However, there are many other binary systems among the near-Earth asteroid population with similar physical and dynamical properties, making the results presented here relevant to the NEA binary population in general.
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    A portrait of the binary compact merger as a young: Short GRB, Gravitational wave, Afterglow, and Kilonova
    (2022) Ahumada Mena, Tomas Fernando; Singer, Leo P; Veilleux, Sylvain; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Both binary neutron star (BNS) and neutron star–black hole (NSBH) mergers radiate gravitational waves (GWs) as they gradually spiral inwards. Once they merge, they emit electromagnetic (EM) radiation that is potentially detectable across the entire EM spectrum, from hours to years after the coalescence. Right after the merger, a short burst of gamma-rays is followed by an hours to days long optical/near infrared (NIR) transient (i.e. kilonova (KN)), which is powered by the decay of the r-process nucleosynthesis elements. Depending on the angle of the gamma-ray burst (GRB) relative to Earth, a seconds to years long afterglow can be detected from radio to X-rays. The EM radiation from these mergers has shed light into different fields of physics and astronomy: they are sources of GWs, a site of rapid neutron-capture process (r-process) nucleosynthesis, and promising standard candles. However only one BNS merger has been studied in detail: AT2017gfo, the EM counterpart to GRB 170817A/GW170817. This thesis focuses on the opticalsearches of these multi-messenger sources using wide field of view (FOV) telescopes. The first chapter of this thesis describes the systematic search for optical counterparts to short gamma-ray bursts (SGRBs). We used the Zwicky Transient Facility (ZTF) to follow-up 10 short duration GRBs detected by the Fermi Gamma- ray Burst Monitor (GBM). We covered areas between 250 and 3000 deg2, and followed-up more than 60 objects, photometrically and spectroscopically. While we did not find a counterpart to a compact binary merger, we used the ZTF magnitude limits (i.e. ∼ 21 mag in the r-band) to compare to SGRB afterglows and KN models, to show that our searches are sensitive to most KN models up to 200 Mpc. However, the majority of SGRB afterglows in the literature have been found at relatively higher redshifts (i.e. mean z ∼ 0.5), making them fainter than our magnitude limits. Moreover, we explore the efficiency of our searches and we determine our searches have probed between redshift 0.16-0.4, depending on the energy models assumed for the SGRBs. Future campaigns can expand the horizon to redshift 0.2-0.7. The second part of this thesis is about the discovery of the shortest gamma-ray burst coming from a collapsing massive star. In the context of the optical follow-up of short GRBs with ZTF, we triggered target-of-opportunity (ToO) observations in the error region of GRB 200826A, a 1.13 sec duration GRB. There we found the afterglow of the burst, ZTF20abwysqy, with an optical decay rate ∼ 1 mag/day. The afterglow was additionally X-ray and radio bright. At the redshift of the host galaxy z = 0.74 , its hardness - intensity relation (i.e. Epeak,z − Eγ,iso) is consistent with the long GRB population, puzzling the community. We present the afterglow and host galaxy analysis, along with Gemini Multi-Object Spectrograph (GMOS) observations that show a rising source in the i-band that could only be explained by an underlying supernova. The third chapter of the thesis describes the optical follow-up of gravitational wave events using the ZTF. We describe the observing strategy, as well as the selection and monitoring of GW counterpart candidates. Our ToO strategy allowed us to sift through ∼ 2 million sources to select ∼ 160 candidates for follow-up. We apply this strategy to search for 13 GW alerts during the third LIGO/Virgo observing run (O3). Particularly, we describe the case of the first BNS merger in O3, S190425z, and two NSBH mergers, S200105ae and S200115j. As no counterpart was found for any of the GW events, we use the photometric limits of our searches to compare to KN models. Finally, we explore how the upcoming Rubin Observatory will be able to serendipitously find KNe, independently from GW or SGRB triggers. For this, we simulated the universe accessible to the survey and use it to derive contamination rates for different classes of transients. When using a filtering scheme based on the magnitude evolution of the sources, we find that ∼90% of the sources that fade faster than 0.4 mag/day are either GRB afterglows or supernova (SN) IIb shock breakout. This strategy is only capable of retrieving ∼3% of the generated KNe, mainly due to the fast fading nature of the KNe and their intrinsic low luminosity. We propose that future filtering schemes should take into consideration not just the detections, but the difference in magnitudes, ∆m, between the last detection and the subsequent limiting magnitude. Additional information as color, host galaxy or NIR counterparts on future NIR surveys could also improve the selection.
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    Tracing the formation and merger-driven growth of massive black holes with the Zwicky Transient Facility
    (2022) Ward, Charlotte Alison; Gezari, Suvi; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The dawning of low--frequency gravitational wave (GW) astronomy via pulsar timing arrays and space--based GW interferometry will provide new opportunities for the study of the supermassive black hole (SMBH) binaries which form as galaxies merge through cosmic time. The onset of observational GW studies has coincided with the expansion of wide--field optical time--domain surveys such as the Zwicky Transient Facility (ZTF), which provide a complementary way to detect and analyze SMBHs when they accrete gas and emit at optical wavelengths. In this thesis I describe how high cadence surveys like ZTF can be used to discover rare populations of massive black holes which inform our understanding of early massive black hole seeding channels and their subsequent growth through mergers to produce the SMBH populations we see today. In the first part of this thesis I present a search for variable active galactic nuclei (AGN) which are spatially offset from their host galaxies using time-resolved imaging data from ZTF and deeper, higher resolution imaging data from the Legacy Surveys. I present a population of 52 variable AGN in merging galaxies in addition to 9 candidates for gravitational wave recoil of remnant SMBHs which may be used to constrain SMBH binary merger rates and spin alignment efficiencies. I also examine the dramatic rebrightening of a previous recoiling SMBH candidate SDSS1133, and conclude from spectroscopic follow--up that it is more likely an outbursting luminous blue variable star. In the second part of the thesis, I present a population of 190 low--mass AGN in dwarf galaxies discovered by their optical or mid--infrared variability in deep ZTF difference imaging and forward--modeled photometry of {\it WISE} image stacks. These intermediate mass black hole (IMBH) candidates can be used to constrain the low--mass end of the $M_{BH}-\sigma_*$ relation and dwarf galaxy occupation fractions in order to better understand the origins of the first massive black holes. Only $9$ candidates from my search had been detected previously in radio, X-ray, and variability searches for dwarf galaxy AGN. I find that spectroscopic approaches to AGN identification would have missed 81\% of my ZTF IMBH candidates and 69\% of my {\it WISE} IMBH candidates, showing the promise of variability searches for discovery of otherwise hidden low--mass AGN. In the third part of this work, I present 299 variable AGN in ZTF which have double--peaked Balmer broad lines from the motion of gas in their accretion disk, increasing the number of known double--peaked emitters (DPEs) by a factor of $\sim$2. DPEs can arise as false positive candidates in both spectroscopic and variability--based searches for SMBH binaries, so it is important to characterize the properties of their spectra and light curves. I find that 16\% of variable broad line AGN in ZTF are DPEs and that $\sim$50\% of the DPEs display dramatic changes in the relative fluxes of their red and blue peaks over long $10-20$ year timescales. I show that a number of DPEs exhibit apparently periodic and chirping signals in the optical and mid--infrared and discuss how this arises naturally from their power spectra. I show that DPE light curves have slightly steeper power spectra than their standard broad line counterparts and are $\sim$1.5 times more likely to have a low frequency turnover. I compare the variability and spectroscopic properties of the ZTF DPE population with the recently discovered inspiraling SMBH binary candidate SDSSJ1430+2303 (ZTF18aarippg) and conclude that the variable velocity--offset broad lines and periodic behavior of ZTF18aarippg are not unusual compared to other DPEs, and it is therefore more likely to be a single AGN rather than an SMBH binary. I conclude this thesis by outlining how the transient detection and image forward--modeling techniques presented in this thesis can be used to find populations of low accretion rate, off--nuclear AGN with the upcoming Legacy Survey of Space and Time at the Vera Rubin observatory in order to produce much better constraints on massive black hole seeding channels and GW recoil rates. I also discuss how these techniques can be applied to new science cases, such as the analysis of strongly gravitationally lensed supernovae and quasars, for cosmological studies with LSST.
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    (2022) Liu, Weizhe; Veilleux, Sylvain; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Feedback from Active Galactic Nucleus (AGN) may play a critical role in the coevolution of galaxies and supermassive black holes (SMBH). Specifically, AGN feedback may quench star formation, suppress baryon-to-dark matter mass ratio, shape galaxy morphology, impact the circumgalactic (CGM)/intergalactic medium (IGM), and regulate SMBH accretion. One of the most important forms of AGN feedback is powerful, quasar/AGN-driven outflows. However, the physical details of these outflows, including their driving mechanism and spatial extent, are still not well constrained. In addition, while these outflows are believed to be effective in massive galaxies, their impact on dwarf galaxies (M⋆ ~10^9.5 M⊙) remains largely unknown. To answer these open questions, my thesis focuses on AGN feedback via quasar/AGN-driven outflows in both ultraluminous infrared galaxies (ULIRGs) and dwarf galaxies with four projects.In the first half of my thesis, I examine the outflows in nearby ULIRGs with two objectives: In Chapter 2, I present a dedicated investigation of the highly ionized, likely pc-scale quasar/AGN winds in a sample of 21 nearby ULIRGs through HST/COS far-ultraviolet (FUV) spectroscopy. Blueshifted Lyα emission is prevalent in the sample, which is probably closely related to the outflowing gas and AGN activity in these objects. Additionally, the Lyα escape fractions tend to be slightly larger in objects with stronger AGN and larger outflow velocities. Highly ionized O VI and N V outflows are detected in a coherently selected, AGN-dominated ULIRG sample for the first time. Together with the results from a matched quasar sample, these outflows show higher incidence rates and larger EW and velocities in X-ray weak sources and sources with high X-ray absorbing column densities, implying that these outflows are radiatively-driven; In Chapter 3, I describe a deep, Chandra imaging spectroscopy study of the nearby ULIRG Mrk 273. The data have revealed a ∼40 kpc×40 kpc X-ray nebula, which is relatively hot and has a super-solar α/Fe abundance ratio. This nebula is most likely heated and metal-enriched by outflows over time. Additionally, the existence of a dual AGN is strongly suggested by the data, and extended 1–3 keV emission are detected, likely related to the AGN photoionized gas and/or outflowing gas. In the second half of my thesis, I turn to look at the AGN-driven outflows in dwarf galaxies: In Chapter 4, I report the results from a dedicated optical integral field spectroscopic study of a sample of eight dwarf galaxies with known AGN and suspected outflows. Fast, kpc-scale outflows are detected in seven of them. The outflows show 50-percentile (median) velocity of up to ∼240 km s^−1 and 80-percentile line width reaching ∼1200 km s^−1, in clear contrast with the more quiescent kinematics of the host gas and stellar components. The kinematics and energetics of these outflows suggest that they are primarily driven by the AGN. A small but non-negligible portion of the outflowing material likely escapes the main body of the host galaxy and contributes to the enrichment of the circumgalactic medium. The impact of these outflows on their dwarf host galaxies is similar to those taking place in the more luminous AGN with massive hosts in the low-redshift universe. In Chapter 5, I discuss the results from a pilot HST/COS spectroscopy program to examine three objects studied in Chapter 4. Blueshifted absorption features tracing fast outflows are detected in two of the three objects. For object J0954+47, the outflow is detected in multiple ions and is much faster than those in star-forming galaxies with similar star formation rates. The outflow velocity exceeds the escape velocity of this system, suggesting that a large fraction of the outflowing gas may escape. The outflow carries significant amount of mass, momentum and kinetic energy, which may transport material out of the galaxy more efficiently than the gas consumption by star formation. The ratio of kinetic energy outflow rate to AGN luminosity of this outflow is at least comparable to the expectation from simulations of AGN feedback.Finally, in Chapter 6, I summarize the main results of the whole thesis, and briefly highlight several future works that may lead to a more comprehensive understanding of AGN feedback in ULIRGs and dwarf galaxies.
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    A Study of Diverse Hot Jupiter Atmospheres
    (2022) Fu, Guangwei; Kempton, Eliza; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The discovery of over 5000 planets outside the solar system has evermore changed our view of the universe and the tale of other worlds beyond Earth is no longer folklore. In 1989, David Latham found the first planetary candidate HD 114762b (Latham et al., 1989) using the radial velocity technique which was later used to discover 51 Pegasi b as the first exoplanet orbiting a solar-type star by Didier Queloz and Michel Mayor (Mayor and Queloz, 1995). Then back to December 1999, the first transiting exoplanet HD 209458b lightcurve was measured by David Charbonneau (Charbonneau et al., 2002). The gold rush of exoplanet discovery and characterization today is probably beyond the wildest dreams of the early pioneers.From the early days of discovering hot Jupiters, we are now able to study their atmospheres in detail. This thesis focuses on the study of hot Jupiter atmospheres. Hot Jupiters are rare outcomes of planet formation and their origin remains a mystery (Dawson and Johnson, 2018). The study of their atmospheres can help us to understand their formation and evolution history and also develop techniques for future remote sensing of potentially habitable transiting exoplanets in the search for life beyond earth. Just like our solar system with every planet being special and different, the same is true for hot Jupiters. I will first go into detailed studies of 4 individual hot Jupiters (WASP-76b, WASP-74b, HAT-P-41b, and KELT-20b) and then connect them to more broad population-level statistical studies. These four hot Jupiters all have dayside temperatures exceeding 2000K with an inflated radius and a very short (<10 days) orbital period. However, they exhibit different atmospheric features and properties. The relatively cooler ones like WASP-74b and HAT-P-41b show mostly blackbody-like dayside emission spectra which indicate isothermal temperature-pressure profiles. The largely featureless transmission spectrum from WASP-74b is likely caused by clouds condensing in the terminator region. On the other hand, the hotter WASP-76b dayside shows CO emission features with evidence for water thermal dissociation. Combined with heavy metal absorption features seen in the NUV part of the transmission spectrum, gaseous metals are likely causing thermal inversion in the upper atmosphere. KELT-20b shows the strongest water and CO emission features on the dayside despite a similar dayside temperature compared to WASP-76b. The unique A-type host star of KELT-20b is likely the difference-maker. The intense short-wavelength UV/NUV radiation from the A star gets preferentially absorbed in the upper atmospheres by the gaseous metals which heats up the corresponding layers and drives stronger thermal inversion. The dayside emission spectra of these four planets are then compared to all other hot Jupiters with temperatures ranging from ∼1500K to over 4000K. These four planets sit in a key transitional parameter space where we see dayside emission spectra from cooler planets mostly have water absorption features and hotter planets are mostly blackbody-like. This trend shows the cooler (<2000K) planets do not have high altitude absorbers needed for thermal inversion, and the much hotter (>3000K) ones are affected by thermal dissociation of water and rising continuum opacity of H−. Only in this in-between temperature range combined with strong inversion from gaseous metal absorbers and strong UV radiation, we could see prominent water emission features.I also did a population-level statistical study of all observed transmission spectra focused on the 1.4 μm water band. Each spectrum was fitted to determine the water feature strength which is then normalized by the planetary atmospheric scale height. I found a statistically significant trend of stronger water absorption features as a function of planet equilibrium temperature. This trend can be explained by the presence of aerosols which condense more easily under cooler conditions. Although this study was conducted back in 2017 before my other publications, the trend still holds and provides a valuable statistical comparison study framework for exoplanet atmospheres.
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    Case Studies in AGN Feedback
    (2022) Smith, Robyn N; Reynolds, Christopher S; Veilleux, Sylvain; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Galaxies in which the central supermassive black hole (SMBH) is actively accreting are referred to as active galactic nuclei (AGN) and are believed to play a crucial role in the evolution of both individual and clusters of galaxies. Empirically, the mass of the host galaxy and the mass of the SMBH are positively correlated. This is somewhat surprising given that the gravitational sphere of influence of the SMBH is orders or magnitude smaller than the host galaxy. The SMBH is believed to undergo periods of activity during which it is capable of powering galactic-scale outflows which in turn modulate star formation and therefore the overall mass of the host galaxy. Such processes are broadly referred to as feedback.Clusters of galaxies are the largest gravitationally bound systems in the universe. The intracluster medium (ICM) in relaxed clusters is strongly centrally peaked and suffi- ciently dense that it is expected to cool rapidly (in cosmological terms). Such cooling should create streams of cool gas flowing to the brightest cluster galaxy (BCG) which in turn should fuel high rates of star formation. Little evidence of either has been found giving rise to the ‘cooling flow problem’. AGN are again invoked to explain the absence of this cooling flow. The BCGs hosting AGN, often with powerful radio jets, are believed to inject energy into the ICM at a rate which can counteract the cooling. This cyclical nature of balancing the cooling is another form of AGN feedback. In this thesis, we present case studies of three AGN which provide unique insight into these feedback processes. Chapter 2 presents evidence for a relativistic X-ray driven outflow on accretion disk scales in an ultraluminous infrared galaxy known to host a galactic-scale molecular outflow. The observational properties which make a galaxy an ideal candidate for detection of large-scale outflows are intrinsically at odd with the properties which are ideal for detecting small-scale outflows. IRASF05189-2524, the subject of Chapter 2, is one of only a handful of galaxies for which positive detection of outflows on both small- and large-scale exist. Next, we turn our attention to AGN in BCGs and the cooling flow problem. Chapter 3 presents new Chandra observations of NGC 1275, the BCG in the famous Perseus Cluster. The high-cadence observing campaign finds X-ray variability on short intraweek timescales. The inclusion of archival observations reveals a general ‘harder when brighter’ trend. Examination of multiwavelength light curves finds a strongly correlated optical and γ-ray flare in late 2015 in which the optical emission leads the γ-ray emission by ~5 days. This robust (> 3σ) result is the first strong evidence of correlated emission with a time delay and is lends support to the idea that the γ-ray emission is produced by synchrotron self-Compton upscattering. In Chapter 4, we present new Chandra observations of the rare radio-quiet BCG quasar H1821+643. It is one of only two examples in the nearby universe of a highly luminous quasar with minimal radio jet activity at the center of a galaxy cluster. Despite observational challenges, we produce the first high-resolution spectrum of the quasar well-separated from the ICM in ~20 years. Our short-cadence observing campaign again reveals rapid variation on timescales corresponding to the light crossing time of the accretion disk. Although the flux varies, the spectrum is remarkably constant when compared to observations from previous decades. The result of this thesis is to add to the existing body of knowledge of AGN feedback on both galaxy and galaxy cluster scales. These three AGN presented various observing challenges which required a combination of non-standard observational techniques and data reduction methods in order to maximize results with current X-ray instrumentation.
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    Investigating Star Formation Feedback Through Gas Kinematics in Nearby Galaxies
    (2021) Levy, Rebecca Chyba; Bolatto, Alberto D; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Many stages of the stellar life cycle release energy and momentum into the surrounding interstellar medium within a galaxy. This feedback can have profound effects on the host galaxy. This thesis investigates the role of stellar feedback in star-forming galaxies in the local Universe through multiwavelength observations of gas kinematics. First, I study extraplanar diffuse ionized gas (eDIG) which is thought to be produced by gas ejected from the midplane by repeated supernova explosions. By comparing molecular and ionized gas rotation curves derived from a sub-sample of intermediate inclination star-forming galaxies from the EDGE-CALIFA Survey, I find that ~75% of my sample galaxies have smaller ionized gas rotation velocities than the molecular gas. I suggest and show that the lower ionized gas rotation velocity can be attributed to a significant contribution from eDIG in a thick disk which rotates more slowly than gas in the midplane. As a direct follow up to this study, I use a sample of edge-on galaxies selected from the CALIFA survey to directly investigate the prevalence, properties, and kinematics of eDIG. I find that 60% of these galaxies show a decrease in the ionized gas rotation velocity as a function of height above the midplane. The ionization of the eDIG is dominated by star-forming complexes. These studies reveal the pervasiveness and importance of this phase in local star-forming galaxies. Next, I study stellar feedback in the extreme environment of the nuclear starburst in the nearby galaxy NGC 253. Using ALMA observations with 0.5 pc resolution, I detect blueshifted absorption and redshifted emission (P-Cygni profiles) in multiple spectral lines towards three of the super star clusters (SSCs). This is direct evidence for outflows of dense molecular gas from these SSCs. Through a comparison of the outflow properties with predictions from simulations, I find that the outflows are most likely powered by dust-reprocessed radiation pressure or O-star stellar winds. The observed outflows will have very substantial effects on the clusters' evolution. Finally, I find that the arrangement of the SSCs may be morpho-kinematically consistent with a ring or crossing streams from the larger-scale gas flows which fuel the starburst.
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    Revealing Unique Exoplanet Atmospheres with Multi-Instrument Space Telescope Transit and Eclipse Spectroscopy
    (2021) Sheppard, Kyle Benensohn; Deming, Drake; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Atmospheres act as windows into their host planets, containing measurable information on their planets' chemistry, climate, and atmospheric physics. The bulk properties of planets outside of the Solar System (exoplanets) prove to be much more varied than the Solar System, allowing the ability to test atmospheric models over a range of temperatures, radii, and host star properties. Modeling and observing exoplanet atmospheres provides a better understanding of both atmospheric processes and planetary diversity, and it places the Solar System in a greater context to understand how unique it is, if at all. I take a broad approach, analyzing both transit and emission spectroscopy of 5 exoplanets populating the edges of parameter space, ranging from cool, Earth-sized planets (T$\sim$500K, R=0.8\rearth{}) up to massive, ultra-hot Jupiters (T$\sim$2500K, M=10\mjup{}). I use my publicly available, open source Python 3 analysis pipeline \texttt{DEFLATE} to process telescope data and produce verifiable spectra. I then retrieve atmospheric properties using a forward model + Bayesian sampler retrieval tool, exploring how both inter- and intra- modeling assumptions impact results. I retrieve unexpected atmospheres, including: evidence of stellar activity mimicking water vapor features in two terrestrial planets in the multi-planet L9859 system; evidence of a clear atmosphere and a superstellar atmospheric metallicity and water abundance ($5\sigma$ detection) in the hot Jupiter HAT-P-41b (R=1.65\rjup{}, T$_{\textrm{eq}}$=1950~K); a potentially non-TiO driven thermal inversion and a photometric CO detection ($6\sigma$) in the ultrahot Jupiter WASP-18b; and a water absorption feature ($2.8\sigma$) and non-inverted T-P profile in the water-dissociation-vulnerable hot Jupiter WASP-19b (R=1.4\rjup{}, T$_{\textrm{eq}}$=2120~K). Overall, these results expand already extensive diversity of exoplanet atmospheres.
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    From Tantrums to Transformations: AGN Transients Discovered with the Zwicky Transient Facility
    (2021) Frederick, Sara; Gezari, Suvi; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This dissertation work has consisted of searches for extreme AGN-related outbursts during Phase I of the Zwicky Transient Facility (ZTF) survey, which has been a ground-breaking wide-field instrument for the real-time detection and regular cadence monitoring of transients in the Northern Sky. Transients found to be nuclear through photometric filtering were vetted by humans and coordinated for prompt follow-up with various rapid robotic, spectroscopic, and high energy resources, to understand the nature of the galaxy centers undergoing flares and the appearance of spectral features. Findings from this unprecedentedly high-volume data stream were often serendipitous, and led to surprising new avenues for study, including 1) the establishment of a new observational class of quiescent galaxies caught turning into quasars, 2) the discovery of a preponderance of smooth and high-amplitude optical transients hosted in NLSy1s, and 3) a framework for distinguishing extreme AGN variability from other transients in AGN. We present the results of these observations, including candidates for TDEs in AGN, changing-look AGN caught "turning-on", as well as members of the new emerging observational class of flares in Narrow-Line Seyfert 1 (NLSy1) galaxies associated with enhanced accretion (Trakhtenbrot et al. 2019). We compared the properties of these samples of flares to previously reported changing-look quasars and Seyfert galaxies, confirmed that they are a unique observational class of transients related to physical processes associated with the central supermassive black hole's accretion state, and considered the observations in the context of the physical interpretations for a range of related transients from the literature. With these unique sample sets, we also aim to understand why we have found certain galaxy types to preferentially host the sites of such rapid enhanced flaring activity, and attempt to map out the innermost environment of the accretion events. These pathfinding studies enabled with ZTF have the potential to guide how these exceptional moments of AGN evolution will be systematically discovered in future large area surveys such as the Vera C. Rubin Observatory.
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    (2021) Cunningham, Virginia Anne; Cenko, S. Bradley; Vogel, Stuart; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Long gamma-ray bursts (GRBs) are produced during the deaths of massive stars. They are the most powerful explosions known in the Universe and release most of their energy via a narrow cone of emission. The long-lived afterglows of the brightest GRBs detected by the \textit{Fermi} Large Area Telescope (LAT) are visible from radio to gamma-rays, and this relative abundance of broadband data makes them excellent tools for constraining theoretical models regarding their origins. Here, we use our sample of bright GRBs to test emission models beyond the canonical on-axis, top-hat jet model which has historically been applied throughout the literature. We demonstrate that many GRBs are likely to produce emission via a structured jet. We also find that derived physical parameters are highly dependent upon the fraction, $\xi$, of electrons which contribute to the synchrotron emission. Our findings for $\xi$ are contrary to what is generally assumed during GRB modeling ($\xi=1.0$), but consistent with theoretical simulations which predict lower values. Lower predictions for $\xi$ would impact our current understanding of GRBs, implying denser environments and higher energetics than commonly assumed. Fast radio bursts (FRBs) are extremely bright, short-duration pulses at radio frequencies that were only confirmed as true astrophysical sources a decade ago. Although the field has experienced major leaps in recent years, many questions regarding their progenitors and emission processes remain. The identification of counterparts at higher energies is critical to understanding the physical origins of FRBs. Here, we report on an archival search of previously identified FRBs with the \textit{Fermi} Gamma-ray Burst Monitor (GBM), the \textit{Fermi}-LAT, and the \textit{Swift} Burst Alert Telescope (BAT). We find no significant X-ray or gamma-ray counterparts but report upper limits on the high-energy fluence, $f_{\gamma}$, for each FRB in our sample. We also report lower limits on the ratio of radio to high-energy fluence ($\nicefrac{f_{r}}{f_{\gamma}}$). We discuss the implications of our results on several FRB progenitor theories, including pulsar-like analogs and magnetar flares.