High-fidelity reaction kinetic modeling of hot-Jupiter atmospheres incorporating thermal and UV photochemistry enhanced by metastable CO(a3Pi)

TL;DR

This paper develops a high-fidelity reaction kinetic model for hot-Jupiter atmospheres that integrates UV photochemistry enhanced by metastable CO, revealing its impact on atmospheric composition and chemistry at high temperatures.

Contribution

It introduces an automated mechanism generator to combine thermochemical and UV photochemical processes, including metastable CO, for exoplanet atmosphere modeling.

Findings

Photochemistry driven by Lyman-alpha photons enhances methane, water, and CO2 formation.

Thermal chemistry dominates above 2000 K, reducing photochemical effects.

No larger organic molecules or PAHs are produced up to 2500 K in the model.

Abstract

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 to 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 (a3Pi). Our model results show that two different photochemical reactions driven by Lyman-a photons (i.e. H2 + CO(a3Pi) -> H + HCO and CO(X1Sig+) + CO(a3Pi) -> 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 with 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 the previous experimental study.