Polycyclic aromatic hydrocarbons in exoplanet atmospheres

I. Thermochemical equilibrium models

¹ Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 München, Germany
e-mail: ddubey@usm.lmu.de
² Exzellenzcluster ‘Origins’, Boltzmannstr. 2, 85748 Garching, Germany
³ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstr. 1, 85748 Garching, Germany
⁴ Fakultät für Chemie und Pharmazie, Ludwig-Maximilians-Universität München, Butenandtstr. 5-13, 81377 München, Germany

Received: 19 May 2023
Accepted: 7 August 2023

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 disks. 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 1D, 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 1D 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 in 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.