TIRAMISU: Non-LTE radiative transfer for molecules in exoplanet atmospheres

TL;DR

The paper introduces TIRAMISU, a new code for on-the-fly non-LTE molecular spectra computation in exoplanet atmospheres, demonstrating its application to hot Jupiter KELT-20 b and highlighting significant effects on atmospheric modeling.

Contribution

It presents the TIRAMISU code for non-LTE radiative transfer calculations and applies it to exoplanet atmospheres, revealing important impacts on molecular opacities and photodissociation rates.

Findings

Non-LTE effects are detectable in mid-infrared and visible wavelengths.

Non-LTE can increase OH photodissociation rates by two orders of magnitude.

Non-LTE conditions can significantly alter molecular opacity and abundance retrievals.

Abstract

The TIRAMISU code, a new program for computing on-the-fly non-LTE molecular spectra and opacities for solving self-consistent radiative transfer problems in exoplanet atmospheres, is presented. The ultra-hot Jupiter KELT-20 b is used as a case study to identify the wavelength regions at which non-LTE effects may be detectable. It is shown that upper atmospheric OH in vibrational non-LTE should be observable primarily via hot bands in the mid-infrared and enhanced photodissociation in the visible. Varying the abundance of OH in non-LTE demonstrates a non-linear relationship between the abundance and the strength of non-LTE effects. Using recent calculations of the photodissociation probabilities of OH it is shown that non-LTE effects can increase the total photodissociation rate by two orders of magnitude in the upper atmosphere, which is likely to have a significant impact on atmospheric and chemical modelling. Increases and reductions in the molecular opacities under non-LTE conditions may lead to the mischaracterisation of molecular abundances in retrievals that only consider opacities computed under LTE. Collisional data requirements to support future non-LTE modelling for a variety of exoplanet atmospheres and across a wide range of wavelengths are discussed.