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

Presentation #327.04 in the session Extrasolar Planets II.

Published onJul 01, 2023
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).

1. Tsai, S.-M. et al. Photochemically-produced SO2 in the atmosphere of WASP-39b. Nature in print (2023).

2. Drummond, B. et al. The carbon-to-oxygen ratio: Implications for the spectra of hydrogen-dominated exoplanet atmospheres. Mon Not R Astron Soc 486, 1123–1137 (2019).

3. Lee, R. A. et al. Laboratory Study of the Cameron Bands, the First Negative Bands, and Fourth Positive Bands in the Middle Ultraviolet 180–280 nm by Electron Impact Upon CO. J Geophys Res Planets 126, 2954–2977 (2021).

4. Roudier, G. M. et al. Disequilibrium Chemistry in Exoplanet Atmospheres Observed with the Hubble Space Telescope. Astron J 162, 37 (2021).

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