Atmospheric Library of Far Away Worlds: Developing a Library of Model Simulations to Interpret Observations of Exoplanet Atmospheres
Date: 7 October 2019
University of Exeter
PhD in Physics
The number of exoplanets that have been discovered now exceeds 3500. This population of exoplanets contains a rich diversity, for example; hot Jupiters, warm Neptunes, super Earths and Earth-like planets, exhibiting variation in the planetary mass, radius, and distance from the host star. Moreover, the diversity of exoplanets is greatly ...
The number of exoplanets that have been discovered now exceeds 3500. This population of exoplanets contains a rich diversity, for example; hot Jupiters, warm Neptunes, super Earths and Earth-like planets, exhibiting variation in the planetary mass, radius, and distance from the host star. Moreover, the diversity of exoplanets is greatly multiplied by the potential variation in characteristics such as the internal and atmospheric composition. However, in terms of observing exoplanet atmospheres hot Jupiters are currently the best targets, due to their high temperatures and large atmospheric scale heights. This makes high spectroscopic signal-to-noise ratio (SNR) observations possible, using telescopes like the Hubble Space Telescope (HST), Very Large Telescope (VLT) and in the future the James Webb Space Telescope (JWST). Therefore, the major focus of this thesis has been to develop theoretical models to interpret the observations of exoplanet atmospheres, primarily hot Jupiters and warm Neptunes. A given planet’s observed transmission or emission is shaped by different phys- ical and chemical processes occurring in the atmosphere. Forward models aid in the understanding of these processes when used in conjunction with observations. Therefore, in this thesis we explore hot Jupiter exoplanet atmospheres using 1D radiative-convective equilibrium models, and create an extensive publicly available library of simulations covering a variety of H2/He dominated exoplanet atmospheres, termed as the “Atmospheric Library of Far Away Worlds”. In this thesis we present three different libraries of model simulations using high temperature line-lists. First, a planet specific library of transmission spectra and equilibrium chemical abundances for 117 observationally significant exoplanets for a range of temperatures, metallicity, C/O ratio, cloud and haze parameters, with isothermal Pressure-Temperature (P-T) profiles. Second, a generic library of transmission spectra and equilibrium chemical abundances, that can be scaled to a wide range of H2/He dominated exoplanet atmospheres for all parameters considered in the first library, along with planetary gravity and a wider temperature range, again using isothermal P-T profiles. The first two libraries were developed using two approaches of condensation, local condensation and condensation with rainout. Finally, a library with self-consistent radiative-convective equilibrium (rce) P-T profiles and corresponding equilibrium chemical abundances, transmission and emission spectra, and contribution functions, using condensation with rainout. Transmission spectra observations have been extremely productive in identifying specific trace species in the atmospheres of some hot Jupiter planets. However, the transmission spectrum of a given planet is a combination of the effect of opacities due to all the species in its atmosphere. Given that the number of chemical species present can be considerable and each have different absorption cross-sections, it is challenging to determine which species is responsible for shaping features within the spectrum itself. Moreover, the opacities and the chemical composition of the atmosphere are a function of its pressure-temperature (P-T) structure, metallicity, carbon-to-oxygen ratio and the condensation processes occurring within the atmosphere. Therefore, a library of model transmission spectra and corresponding chemical abundances will therefore aid identifying the species contributing substantially to the transmission spectra and thereby help constrain the target atmospheric chemical composition. Recent transmission spectra observations of hot Jupiter exoplanet atmospheres have shown that their spectral features, especially H2O, are smaller in amplitude than that expected for a clear atmosphere with solar metallicity. What causes these spectral features to mute is an open question, with the presence of clouds and haze as one possibility or high mean molecular mass. A transmission spectrum library can aid in identifying different mechanisms that could cause these spectral features to mute. We have used the planet specific library of model simulations to interpret transmission spectra observations of twelve exoplanet atmospheres and characterise them. We also demonstrated the application of this library in conjunction with the JWST simulator PandExo, as a predictive tool to plan future observations. The planet specific library includes models with fixed gravity for a particular planet. Therefore, to make the library of models flexible enough to be adapted to any gravity values (different planets) and also to their updated values in the future, we developed the generic library of transmission spectra and derive a scaling relationship to scale them to a range of H2/He dominated exoplanet atmospheres. The generic library spans a wider temperature range and is also not constrained by the planetary equilibrium temperature, unlike the planet specific library. Therefore, generic library can reveal important processes due to temperature anomalies, for example, the presence of VO without TiO , in the atmosphere of hot Jupiter WASP-121b. The weather, climate and dynamics of a planetary atmosphere are governed by its pressure-temperature (P -T ) structure. Therefore, it is necessary to constrain the P -T structure of a planet’s atmosphere, to understand the various physical processes occurring within them. The presence of an atmospheric temperature inversion can also be determined by constraining the P-T structure. The P-T structure of the planetary atmosphere also governs its spectral signatures, when remotely observed. Transmission spectrum probe a very small vertical region and only the limb of the planetary atmosphere and cannot constrain its P -T structure. This can be overcome by measuring the emission spectrum of the planet, where the flux originates from much deeper in the planet’s atmosphere as well as from the complete planetary hemisphere. Moreover, the emission spectrum can be used to constrain the P-T structure of the planetary atmosphere. Therefore, to overcome the limitations due to the assumption of isothermal P-T profiles, and also to generate planetary model emission spectra, we developed a third library with radiative-convective equilibrium P-T profiles. We investigated the effect of various model choices such as condensation methodology, treatment of line-wing profiles, source of line-lists, convection and the methodology to vary the C/O ratio, on the P -T profiles and thereby the spectra. We showed how thermal ionisation, H-, Fe and TiO/VO opacities shape the P-T structure of extremely irradiated hot Jupiters like WASP-121b. We investigated different mechanisms which act to form a temperature inversions and their observable signatures in the emission spectra. This library of model simulations with radiative- convective equilibrium P-T profiles has also been used to discover the temperature inversion for the first time in an exoplanet atmosphere in WASP-121b. Simulated observations using theoretical forward models also aid in predicting scientifically important targets for characterisation, to use precious telescope time most efficiently. The launch of the JWST will enable probing exoplanet atmospheres from wavelengths of 0.6 μm all the way up to 28 μm. Therefore, all the libraries of models presented in this thesis will also be extremely valuable to select the best targets for characterisation using the JWST.
Item views 0
Full item downloads 0