Spectroscopic model characterisation of hot exoplanet atmospheres

Abstract: Hot Jupiters are gas giants under intense stellar radiation with short orbital periods of only a few days. Due to their large radii, hot temperatures, and large scale heights, hot Jupiters can be observationally characterised in detail through spectroscopy over an entire orbital phase. Transmission spectroscopy is one of the tools that aid us in understanding the complex chemistry of exoplanet atmospheres. Observations are not the only method of estimating atmospheric compositions. For example, we can use advanced codes to model an exoplanet’s chemical compositions and radiative transfer. However, advanced models are computationally expensive. When performing abundance retrievals where you have to create hundreds of thousands of model templates, we need approximations that are fast enough for retrieval algorithms. In this project, we have used the semi-analytical code FastChem to model atmospheric chemical composition and petitRADTRANS to model the radiative transfer. From petitRADTRANS, we can model transmission spectra for different planetary parameters and choose whether we want a constant abundance throughout the atmosphere and whether we want to include variable gravity or not. We then wish to compare these solutions for different species with a fast analytical approximation. Heng & Kitzmann (2017) derived an analytical solution for the transit radius, which assumes an isothermal and isobaric atmosphere. They tested it for the WFC3 water band between 1.15-1.65 μm for a planet with a temperature of 1500 K. We wish to see whether it still holds for higher temperatures and other species. The analysis has been performed for temperatures of 1500 K, 2500 K, and 3500 K, for the hot Jupiter HD 209458b. We have investigated H2O, CO, Fe, Fe II, Ti, V, Mg, and Cr. These are interesting when studying ultra-hot Jupiters as they become detectable when the temperature is high. We found that the analytical approximation by Heng & Kitzmann (2017) works remarkably well for the species when the temperature is 1500 K. However, once we increase the temperature, we find that the approximation usually underestimates the spectral line strengths. For H2O, it instead overestimates the spectral line strengths. The analytical approximation by Heng & Kitzmann (2017) would benefit from including mass fractions, gravity, and mean molecular weight, which all vary with pressure and temperature for each atmospheric layer. However, the more we expand the approximation to improve its accuracy, the more computationally expensive it becomes. We need these fast models for retrieval algorithms, and there must be a balance between the approximation’s accuracy and its computational speed. We conclude that we must be careful when using the Heng & Kitzmann (2017) approximation and ensure that our application of the approximation is logical and within the scope of its capabilities. We must proceed with caution when analysing ultra-hot Jupiters, as the approximation’s accuracy quickly deteriorates as we approach high temperatures. This is especially true for species such as Fe II that have a mass fraction that increases with altitude. Furthermore, we find that the approximation does poorly to varying degrees for different species. Therefore, it should not be used to perform relative abundance retrievals, especially for ultra-hot Jupiters.