Astronomy Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2746
Browse
3 results
Search Results
Item 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.Item 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.Item 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.