Polycyclic Aromatic Hydrocarbons in Exoplanet Atmospheres I. Thermochemical Equilibrium Models

Abstract

Context: Polycyclic Aromatic Hydrocarbons, largely known as PAHs, are widespread in the universe and have been identified in a vast array of astronomical observations from the interstellar medium to protoplanetary discs. They are likely to be associated with the chemical history of the universe and the emergence of life on Earth. However, their abundance on exoplanets remains unknown. Aims: We aim to investigate the feasibility of PAH formation in the thermalized atmospheres of irradiated and non-irradiated hot Jupiters around Sun-like stars. Methods: To this aim, we introduced PAHs in the 1-D self-consistent forward modeling code petitCODE. We simulated a large number of planet atmospheres with different parameters (e.g. carbon to oxygen ratio, metallicity, and effective planetary temperature) to study PAH formation. By coupling the thermochemical equilibrium solution from petitCODE with the 1-D radiative transfer code, petitRADTRANS, we calculated the synthetic transmission and emission spectra for irradiated and non-irradiated planets, respectively, and explored the role of PAHs on planet spectra. Results: Our models show strong correlations between PAH abundance and the aforementioned parameters. In thermochemical equilibrium scenarios, an optimal temperature, elevated carbon to oxygen ratio, and increased metallicity values are conducive to the formation of PAHs, with the carbon to oxygen ratio having the largest effect.