James Webb Space Telescope Feed Post

Harvard ADS: Ab initio quantum dynamics as a scalable solution to the exoplanet opacity challenge: A case study of CO_2 in hydrogen atmosphere

Paper abstract: Light-matter interactions lie at the heart of our exploration of exoplanetary atmospheres. Interpreting data obtained by remote sensing is enabled by meticulous, time- and resource-consuming work aiming at deepening our understanding of such interactions (i.e., opacity models). Recently, \citet{Niraula2022} pointed out that due primarily to limitations on our modeling of broadening and far-wing behaviors, opacity models needed a timely update for exoplanet exploration in the JWST era, and thus argued for a scalable approach. In this Letter, we introduce an end-to-end solution from ab initio calculations to pressure broadening, and use the perturbation framework to identify the need for precision to a level of ~10\%. We focus on the CO_2-H_2 system as CO_2 presents a key absorption feature for exoplanet research (primarily driven by the observation of gas giants) at ~4.3\mum and yet severely lack opacity data. We compute elastic and inelastic cross-sections for the collision of {ortho-}H_2 ~with CO_2, in the ground vibrational state, and at the coupled-channel fully converged level. For scattering energies above ~20~cm^{-1}, moderate precision inter-molecular potentials are indistinguishable from high precision ones in cross-sections. Our calculations agree with the currently available measurement within 7\%, i.e., well beyond the precision requirements. Our proof-of-concept introduces a computationally affordable way to compute full-dimensional interaction potentials and scattering quantum dynamics with a precision sufficient to reduce the model-limited biases originating from the pressure broadening and thus support instrument-limited science with JWST and future missions.