Implementation of disequilibrium chemistry to spectral retrieval code ARCiS and application to sixteen exoplanet transmission spectra: Indication of disequilibrium chemistry for HD 209458b and WASP-39b

TL;DR

This study enhances exoplanet atmospheric spectral retrievals by incorporating disequilibrium chemistry effects, revealing significant disequilibrium signatures in several exoplanets, notably HD 209458b, and demonstrating the importance of considering chemical quenching in spectral analysis.

Contribution

The paper introduces a disequilibrium chemistry module into the ARCiS spectral retrieval code, enabling more accurate atmospheric composition analysis of exoplanets.

Findings

Indications of disequilibrium chemistry in HD 209458b and WASP-39b with high significance.

Detection of a strong NH3 absorption feature in HD 209458b's spectrum due to quenching effects.

Differences in C/O ratios and other parameters when comparing disequilibrium and equilibrium retrievals.

Abstract

The retrieval approach is currently a standard method for deriving atmospheric properties from observed spectra of exoplanets. However, the approach ignores disequilibrium chemistry in most current retrieval codes, which can lead to misinterpretation of the metallicity or elemental abundance ratios of the atmosphere. We have implemented the disequilibrium effect of vertical mixing or quenching for the major species in hydrogen/helium-dominated atmospheres, namely $\mathrm{CH_4}$, $\mathrm{CO}$, $\mathrm{H_2O}$, $\mathrm{NH_3}$, $\mathrm{N_2}$, and $\mathrm{CO_2}$, for the spectral retrieval code ARCiS with a physical basis using the chemical relaxation method. Then, using ARCiS updated with this module, we have performed retrievals of the observed transmission spectra of 16 exoplanets with sizes ranging from Jupiter to mini-Neptune. As a result, we find indications of disequilibrium chemistry for HD 209458b ($\geq 4.1\sigma$) and WASP-39b ($\geq 2.7\sigma$). The retrieved spectrum of HD 209458b exhibits a strong $\mathrm{NH_3}$ absorption feature at 10.5 $\mu$m accessible by JWST owing to an enhanced abundance of $\mathrm{NH_3}$ due to the quenching effect. This feature is absent in the spectrum retrieved assuming equilibrium chemistry, which makes HD 209458b an ideal target for studying disequilibrium chemistry in exoplanet atmospheres. Moreover, for HAT-P-11b and GJ 436b, we obtain relatively different results than for the retrieval with the equilibrium assumption, such as a $2.9\sigma$ difference for the C/O ratio. We have also examined the retrieved eddy diffusion coefficient, but could not identify a trend over the equilibrium temperature, possibly due to the limits of the current observational precision.