Unveiling High-Energy Star-Planet Interactions: Ionization Driven Chemistry and its Observability

Abstract (English)

High-energy stellar radiation (EUV and X-rays, collectively XUV) plays a crucial role in Star-Planet Interactions (SPI), profoundly shaping the chemical composition and evolution of exoplanetary atmospheres. In this work, we present atmospheric models computed with cp_zephyr, a newly developed 1D chemical kinetics and photochemistry code designed to robustly simulate highly irradiated environments. cp_zephyr explicitly accounts for vertical mixing, photochemistry, and features a highly detailed treatment of ionization. Specifically, the total ionization rate is calculated as the sum of primary ionization (driven by incident XUV photons) and secondary ionization (driven by non-thermal photoelectrons generated during primary events).

Our results demonstrate that the rich chemistry induced by the ionization of H2 via these non-thermal electrons  drive a substantial enhancement in the abundances of key hydrocarbon and nitrogen-bearing species, including CH4, C2H2, and HCN. Because these molecules exhibit strong and distinct absorption features, they serve as potential chemical tracers of stellar XUV irradiation. We evaluate the observability of these XUV-induced chemical signatures assessing the capability of current and future state-of-the-art facilities, such as the JWST and the Ariel space mission, to detect these characteristic spectral features and ultimately constrain the high-energy environment of exoplanetary systems.