Metastable CO(a³Π)-aided photochemistry in H₂- or N₂-dominated exoplanet atmospheres

The recent study using multiple atmospheric photochemical modeling of WASP-39b suggested the first evidence of photochemistry in an exoplanet atmosphere ¹. As such, photochemistry plays a significant role in determining the composition of exoplanet atmospheres, which provides the rationale that experiment and modeling combined studies on exoplanet atmospheric photochemistry are essential for both interpreting and predicting observational data from various space telescopes such as JWST.

Among a variety of molecules that exist in exoplanet atmospheres, carbon monoxide (CO) is one of major interest to astrochemists for following reasons: (i) CO is predicted to be one of the most abundant species in exoplanet atmospheres whose temperatures are higher than 1000 K ²; (ii) CO can serve as a chemical precursor for more complex organic species; and (iii) electronically excited CO can remain chemically reactive with a long lifetime (Lee et al., 2021), to initiate these photochemical pathways ⁴. Despite its potential importance, the photochemistry of CO and its interaction with other molecules is still poorly understood.

In this work, we utilized an automatic chemical reaction mechanism generator that chooses reaction rates for the target system based on a rate-based iterative algorithm and constructed a high-fidelity chemical model to assess the chemical importance of this photo-excited carbon monoxide in various systems, such as H₂-dominated or N₂-dominated exoplanet atmosphere-like condition. Then we compared our chemical model to corresponding laboratory experiments (i.e., high temperature and UV photochemistry simulating exoplanet atmospheres) to evalulate in detail the photochemical formation pathways to CH₄ or HCN that are led by Lyα-aided electronically excited carbon monoxide in its metastable state (a³Π).

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