published_or_final_version / abstract / Physics / Master / Master of Philosophy

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Thesis (M. Phil.)--University of Hong Kong, 2007. / Title proper from title frame. Also available in printed format.

Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1992. / Vita. Abstract. Includes bibliographical references (leaves 76-81). Also available via the Internet.

Since the discovery of the first exoplanet orbiting a sun-like star in 1995, the fundamental questions as to the formation of our Solar System have met a paradigm shift. The presence of hot Jupiter exoplanets, Jupiter sized worlds rapidly orbiting their host stars, was unlike anything previously seen or predicted. The later discovery of these strange new worlds transiting their stars opened up a new realm of studies into their atmospheres using transit spectroscopy to separate the signals between the star and planetary atmosphere. This thesis investigates the transmission spectral properties of hot Jupiter exoplanets through observations and theoretical analysis from the search for H2O in the near-IR to the signatures of cloud condensates in the IR. Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) observations of transiting hot Jupiters were used to investigate the atmospheric composition over water bands in the near infrared. We put forward a new analysis method to treat the varying systematics seen across transit datasets in a consistent and robust way, in which we marginalise over a grid of possible systematic models used to correct the lightcurves, with each model contributing to the extracted spectrum based on its statistical likelihood. We apply this new method to five previously studied hot Jupiter exoplanet transmission spectra and make direct comparisons between the planetary atmospheres. An apparent dichotomy emerges between two possible sub-classes of hot Jupiter atmospheres with clouds and hazes playing a key role. WFC3 appears to cover a critical wavelength range in exoplanet atmospheres where clouds and hazes potentially obscure the expected molecular signatures in systems where they are found to be obscured in the optical. Using analytical models following Mie theory, we explore the potential atmospheric transmission spectral signatures that would be caused by a variety of cloud condensates in hot Jupiter atmospheres. We find that the observed optical slope representing Rayleigh scattering at high altitudes can constrain the cloud condensate particle size and can be used as a diagnostic for potential condensate features in the IR where almost all condensate absorption features occur. We find that the major transmission spectral absorption features are generated by the vibrational modes of the major diatomic bond pair in each condensate species, which is often seen in the IR at 5-25 microns, and explore the potential for future JWST investigations using MIRI.

Exomoons remain amongst the most elusive targets in observational astronomy. Nevertheless, these worlds stand to provide an unprecedented window into the formation and evolution of planetary systems. If the Solar System is any guide, we can expect exomoons will be geologically active and diverse, with the potential for hosting volatiles, atmospheres, and even life. Moreover, a thorough understanding of the population and occurrence rates of exomoons will help to place our own Solar System in a galactic context, speaking to the commonality of our own history. And though there are a variety of known pathways for moon formation, the discovery of exomoons may yet reveal heretofore unanticipated system architectures and defy easy explanation, thereby enriching our theoretical understanding of system formation. In this Dissertation I present a population study of exomoons in the Kepler data, finding an apparent dearth of Galilean-analog satellites orbiting planets between 0.1 and 1 AU. I then present evidence for a large exomoon orbiting Kepler-1625b -- potentially the first ever discovery of a transiting exomoon -- as suggested by a joint analysis of Kepler and Hubble Space Telescope data. The following chapter further investigates a number of alternative hypotheses relating to the candidate moon, though the conclusion that an exomoon best explains the data in hand remains unchanged. Finally, I present the results of an effort to identify candidate exomoon signals in the Kepler data by developing a convolutional neural network trained on O(10⁵) Kepler light curves injected with simulated planet and moon transit signals. The most promising exomoon candidates identified by the neural network are examined in detail, undergoing a full photodynamical model fit and Bayesian model selection. I conclude by discussing the outlook for the moon search, highlighting strategies for future work and myriad unanswered questions that should be pursued in the coming years.

This study aims to investigate the similarities in precision and performance between a James WebbSpace Telescope mirror actuator, the part controlling the motion of the primary mirror segments,and two 3D printed replicas. The two replicas were made from the plastics PLA and PETGand based on the designs of Zachary Tong. The plastics were examined concurrently and theresults were compared with simulations in COMSOL. The 3D printed parts were assembled and theresulting replicas were put in motion by an Arduino driven stepper motor. The result showed thatthe replica made from PLA had an average precision of 40 nm and the replica made from PETGhad an average precision of 32 nm. Compared to the James Webb Space Telescope actuator, bothreplicas were approximately one order of magnitude less accurate. The COMSOL simulations gavesimilar results. In conclusion, the study shows that the choice of material matters. The performanceof the James Webb Space Telescope actuator was more accurately emulated by the replica madefrom PETG than by the one made from PLA.

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Jupiter's aurorae put on a permanent, ever-changing light show more than a thousand times brighter than the Earth's own aurorae. At ultraviolet wavelengths these aurorae are dominated by the ME: discontinuous ovals of curtain-like light partially encircling each of the planet's magnetic poles. The properties of these aurorae are a reflection of processes in Jupiter's magnetosphere, as the two are coupled together by currents flowing along magnetic field lines. By understanding auroral features in the ME, the vast Jovian magnetosphere's complex interactions with the planet can thus be better understood. The evolution of this energetic system has implications for Jupiter's present and past, as well as its place within the Solar System. While Jupiter's large-scale aurorae have been extensively studied, the properties, particularly motions, of small-scale auroral features represent a comparatively unexplored route to gain deeper understanding of this system. Here, the motions of these auroral features are characterized and related back to the physical processes in Jupiter's magnetosphere and ionosphere. First, a survey of auroral feature motions in Jupiter's ME is created based on Hubble Space Telescope observations. A dichotomy in auroral motion is found: features near dawn remain fixed in local time significantly more than features elsewhere. This finding gives context for Jupiter's dawn storms-- rare, enigmatic auroral phenomena noted for their fixedness, brightness, and appearance only at local dawn. Next, the ME is measured on smaller scales and compared with in-situ measurements of magnetospheric plasma flow from the Galileo spacecraft to estimate the magnetospheric and ionospheric properties associated with fixed auroral features. Finally, these properties are used to inform a self-consistent model of the currents generating the ME. Ionospheric conductance-- the ease with which currents flow through the ionosphere-- is varied to generate models which best match the auroral observations. Altogether, a coherent description of ME auroral features and their associated physical processes emerges. Increased conductance is found to correspond with both auroral emissions and the acceleration of magnetospheric plasma. The conductance, which is spatially variable but fixed in local time on average, is proposed to explain the motions of small-scale ultraviolet Jovian auroral forms.

We present a detailed study of mid-infrared spectroscopy and optical imaging of Seyfert galaxies with the goal of understanding the properties of astronomical dust around the central supermassive black hole and the accretion disk. Specifically, we have studied Spitzer Space Telescope mid-infrared spectra of 12 Seyfert 1.8-1.9s and 58 Seyfert 1s and 2s available in the Spitzer public archive, and the nuclear dust morphology in the central 500 pc of 91 narrow and broad-line Seyfert 1s using optical images from the Hubble Space Telescope. We have also developed visualization software to aid the understanding of the geometry of the central engine. Based on these studies, we conclude that the nuclear regions of Seyfert galaxies are fueled by dusty spirals driven by the large-scale stellar bars in the host galaxy. The accumulation of dusty gas in the central kiloparsec leads to enhanced star formation. In this case, the circumnuclear starburst and the central engine compete for dominance in the heating of the circumnuclear dust. Emission from the heated dust is most clearly seen in the mid-infrared. We find that the spectra of Seyfert 2s show the most variety in the continuum shapes due to different starburst contributions. We find that the spectra of Seyfert 2s that are devoid of starburst contribution are dominated by a single thermal component at a temperature of T ~ 170 K. We also find that the mid-IR continua of Seyfert 1.8/1.9 galaxies are more like those of starburst-dominated Seyfert 2s than Seyfert 1s, contrary to expectations. We discuss the implications of these findings in the context of the Unified Model of AGN and the secular evolution of Seyfert nuclei.