High-fidelity Reaction Kinetic Modeling of Hot-Jupiter Atmospheres Incorporating Thermal and UV Photochemistry Enhanced by Metastable CO(a3II)

detailed modeling of simultaneous UV-photochemical and thermochemical processes in exoplanet atmospherelike 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 (a(3)II). Our model results show that two different photochemical reactions driven by Lya photons (i.e., H-2 + CO(a(3) II). H + HCO and CO(X-1 Sigma(+)) + CO(a(3)II) -> C(P-3) + CO2) can enhance thermal chemistry resulting in significant increases in the formation of CH4, H2O, and CO2 in H-2-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 C-3 species, benzene, or larger (i.e., PAHs). This might indicate that the photochemistry of C-2 species such as C2H2 might play a key role in the formation of organic aerosols observed in a previous experimental study.

Automated summary

Detailed modeling is essential for analyzing spectral data from exoplanets, but the incorporation of simultaneous UV-photochemical and thermochemical processes is challenging. In this study, we used an automatic reaction mechanism generator to build a high-fidelity thermochemical model with enhanced UV photochemistry. Our results showed that photochemical reactions driven by Lya photons can enhance thermal chemistry, leading to the formation of CH4, H2O, and CO2 in H-2-dominated systems with trace amounts of CO. Our model also suggested that above 2000 K, thermal chemistry becomes dominant.