2023ApJ...947...26Y

A detailed modeling of simultaneous UV-photochemical and thermochemical processes in exoplanet atmosphere-like conditions is essential for the analysis and interpretation of a vast amount of current and future spectral data from exoplanets. However, a detailed reaction kinetic model that incorporates both UV photochemistry and thermal chemistry is challenging due to the massive size of the chemical system as well as the lack of understanding of photochemistry compared to thermal-only chemistry. Here, we utilize an automatic chemical reaction mechanism generator to build a high-fidelity thermochemical reaction kinetic model later then incorporated with UV photochemistry enhanced by metastable triplet-state carbon monoxide (a³Π). Our model results show that two different photochemical reactions driven by Lyα photons (i.e., H₂ + CO(a³Π) → H + HCO and CO(X¹Σ⁺) + CO(a³Π) → C(³P) + CO₂) can enhance thermal chemistry resulting in significant increases in the formation of CH₄, H₂O, and CO₂ in H₂-dominated systems with trace amounts of CO, which qualitatively matches the observations from previous experimental studies. Our model also suggests that at temperatures above 2000 K, thermal chemistry becomes the dominant process. Finally, the chemistry simulated up to 2500 K does not produce any larger species such as C₃ species, benzene, or larger (i.e., PAHs). This might indicate that the photochemistry of C₂ species such as C₂H₂ might play a key role in the formation of organic aerosols observed in a previous experimental study.