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Improving Atmospheric Models for Hot Jupiters and sub-Neptunes with New ab initio Reaction Rates

Presentation #213.05 in the session “Exoplanets and Systems: Giant Planet Atmospheres 2”.

Published onOct 26, 2020
Improving Atmospheric Models for Hot Jupiters and sub-Neptunes with New ab initio Reaction Rates

Existing atmospheric models for hot transiting exoplanets can be classified into two general groups: chemical-equilibrium and chemical-kinetics. Chemical-equilibrium models assume thermochemical equilibrium, and seek to minimize the Gibbs free energy, ΔG, of the system and do not require a knowledge of its chemical pathways. On the other hand, chemical-kinetics models incorporate dynamic transport processes like eddy-diffusion, employ an extensive chemical network that includes reaction rates, and require the solution of a large set of differential equations. Any hot exoplanet atmosphere is expected to approach thermochemical equilibrium in its lower atmosphere where both temperature and pressure are high, but various disequilibrium processes like transport-induced quenching and photochemistry become significant in the middle/upper atmosphere where lower temperature and pressure prevail. Therefore, an accurate and comprehensive modeling of hot exoplanet environments requires inclusion of both chemical equilibrium and chemical kinetics processes. Although thermochemical equilibrium is a reasonable starting point for exoplanet composition predictions, disequilibrium processes initiated by the host star’s strong UV flux can perturb the observable upper atmosphere and significantly affect the atmospheric model abundance results. Recent sensitivity analysis study (Tsai et al., 2017) has shown that some key production/loss reaction rates strongly affect methane (CH4) and acetylene (C2H2) molar abundance predictions in hot Jupiter kinetic models. Currently, a number of key reaction rates relevant to hot Jupiter environments are either missing or are largely inconsistent when compared between multiple experimental and theoretical sources, thus resulting in several orders of magnitude errors in model predictions. To address this issue, we are applying high-level ab initio quantum chemical methods to calculate accurate rate data for selected key reactions over a range of temperatures (500-3000 K) and pressures (0-100 bars). Next, our ab initio results will be fed into an existing WASP-12b photochemical model (Kopparapu et al., 2012) to assess the impact of our rate data on the CH4 and C2H2 model abundance predictions. Our calculated ab initio reaction rates will fill an existing void for accurate high-temperature kinetics data and improve the accuracy of all kinetic models for hot Jupiters and sub-Neptunes.

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