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Title: New avenues in atmospheric modelling of exoplanets
Author: Gandhi, Siddharth Nandkumar
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2020
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In this thesis I explore various aspects of atmospheric characterisation of exoplanets with the primary goal of understanding their chemical compositions and physical processes. My research led to the development of new self-consistent models of exoplanetary atmospheres, a new paradigm for atmospheric retrievals of thermal emission spectra, as well as chemical detections using both high-resolution Doppler spectroscopy as well as low-resolution transit spectroscopy. I firstly computed the molecular and atomic cross sections of various species prevalent in the atmospheres of such exoplanets in order to compute their spectra. The absorption cross sections were calculated through the broadening of spectral lines obtained from high resolution line lists. These cross sections and subsequent spectral models have led to the detections of numerous chemical species (HCN, TiO, Li, Na, K, CO, and H₂O) in the atmospheres of several exoplanets. Recent advances in observations have heralded the need for accurate models of exoplanetary atmospheres. I have built a new self-consistent atmospheric model, GENESIS, custom built for exoplanets and demonstrated for irradiated and non-irradiated atmospheres over a wide range of atmospheric parameter space. The model treats line-by-line radiative transfer through the Feautrier method and radiative-convective equilibrium through the Rybicki Complete Linearisation method in a plane parallel atmosphere. This model allows for a detailed exploration of radiative processes and chemical compositions and their effects on observed emission spectra. I compared this model against several others in the literature and found good agreement between the atmospheric properties and emission spectra. Thermal inversions have been seen on the dayside atmospheres of some hot Jupiters and have been predicted to be caused by TiO or VO due to their visible opacity. I used the GENESIS model to investigate the effect of visible opacity and deduced that many new species (AlO, CaO, NaH and MgH), hitherto unexplored, are also capable of causing thermal inversions on hot Jupiters. I have explored the effect of these species as a function of their overall atmospheric abundance as well as determining the required abundance for each of these species to form an inversion. Secondly, I show that a low infrared opacity caused by a low H₂O abundance can also lead to strong thermal inversions even with sub-solar abundances of these visible absorbers due to the change in infrared opacity. As a demonstration of this work I have shown that the thermal inversion on WASP-121b can be explained by all the visible absorbers listed above. These thermal inversions are of great importance as the species responsible may be observed with current observational capabilities, thus providing testable observations for these species. I have also developed a new hybrid retrieval method for exoplanetary emission spectra, HyDRA. This uses the latest atmospheric modelling tools to fit the observed spectra of exoplanet atmospheres. We explore a wide range of parameter space and determine the temperature profile and abundances of various species present in the dayside atmosphere through the emission spectra. These retrieved abundances are then used to explore disequilibrium processes which may be present through integration into the GENESIS self-consistent model. Such a framework allows constraints on departures of the temperature structures from radiative-convective equilibrium as well as chemical compositions from thermochemical equilibrium. I explored HST and Spitzer observations of WASP-43b and confirmed the data were in agreement with radiative-convective equilibrium in the dayside atmosphere. The HyDRA retrieval framework has also been extended to model the atmospheres of ultra-hot Jupiters with temperatures in excess of 2500~K. Such high temperatures can cause molecular species such as H₂O to thermally dissociate and for ionic species such as H- to form. Such effects have been used to explain the largely featureless WFC3 spectra seen for many ultra-hot Jupiters. I have included both of these effects into the HyDRA retrieval model to retrieve the atmosphere of the planet WASP-18b. I find that the retrieved abundances for H₂O and CO and the thermal inversion in the atmosphere do not change significantly compared to previous retrievals of WASP-18b which did not include thermal dissociation or H- opacity. I also see no significant evidence for H- or thermal dissociation in the atmosphere. With future instrumentation we may be more likely to constrain such effects in the emission spectra. I have also used the HyDRA retrieval framework to perform a set of homogeneous retrievals for eight well known hot Jupiters with high precision HST WFC3 spectra. These planets all also have Spitzer observations which I also use to explore the atmospheric temperature profile and chemical composition, in particular explore the H₂O abundance. The eight explored planets span a wide range of equilibrium temperatures, including four which fall into the category of ultra-hot Jupiters. We find that the coolest planets in the study generally have better constrained H₂O abundances near solar composition due to strong H₂O absorption features. On the other hand, three of the hottest exoplanets exhibit thermal inversions and indicate very poorly constrained or sub-solar H₂O abundances. This study shows that even currently explored exoplanets exhibit a wide range of atmospheric properties and that we will be able to explore this diversity further with more exoplanetary spectra coming up in the next few years. Finally, I have used the GENESIS model to enable chemical detections of molecular species using high resolution Doppler spectroscopy of hot Jupiters. I generated high resolution emission spectra of the hot Jupiters HD189733b and HD209458b for cross correlation with the data obtained with the VLT CRIRES spectrograph. This helped us find evidence for H₂O, CO and HCN in the atmosphere of both planets. In the future this method has great potential for new chemical detections due to its sensitivity to trace species and shows great promise in the detection of biosignatures on smaller rocky planets.
Supervisor: Madhusudhan, Nikku Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Exoplanets ; Atmospheres ; Modelling