Wright, Sam O. M.; (2024) Beyond the equilibrium assumption: towards non-LTE analysis of exoplanet atmospheres. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

As a field of research, the study of exoplanets is advancing rapidly both in breadth and depth; since the relatively recent discovery of the first exoplanet, thousands have been detected and great progress has been made in their characterisation, including the analysis of their atmospheres. This pace of advancement is unlikely to diminish, with large surveys and more detailed analysis on the short-term horizon. Improving the detail of exoplanet atmospheric analysis is increasingly important as high resolution spectroscopic instruments come online and start to produce observational data. One key means of adding such detail is via the inclusion of non-local thermodynamic equilibrium (non-LTE) into analyses; the assumption that atmospheres are completely in local thermodynamic equilibrium (LTE) has provided a convenient means of simplifying atmospheric modelling but it differs from reality. In the real scenarios seen on Earth and in the solar system, non-LTE plays a significant role in some spectral features. As a result, its inclusion is necessary to correctly infer atmospheric parameters including species abundances. Unfortunately, limitations exist in applying full non-LTE modelling approaches to exoplanet atmospheres. Data prerequisites necessary for calculating species state populations are prohibitive; collisional and reaction rate data is not available for all species and conditions. These limitations have driven the pragmatic approaches to non-LTE analysis presented in this thesis; they are empirically motivated and aim to take non-LTE into account, without calculating full state populations. One such approach is a two temperature approximation, which is implemented in a forward model and for which non-LTE spectral signatures are shown. Another is the individual vibrational band analysis of the atmosphere of WASP-33b; by isolating individual vibrational bands, LTE is not a required assumption. This thesis presents methodologies for simple treatments of non-LTE in exoplanet atmospheres and shows the results of analyses conducted with these methods.

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