Implementation of disequilibrium chemistry to spectral retrieval code ARCiS and application to 16 exoplanet transmission spectra

Abstract

Context. 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 a misinterpretation of the metallicity or elemental abundance ratios of the atmosphere. Aims. We have implemented the disequilibrium effect of vertical mixing or quenching for the major species in hydrogen and helium-dominated atmospheres, namely CH4, CO, H2O, NH3, N2, and CO2, for the spectral retrieval code ARCiS with a physical basis. Methods. We used the chemical relaxation method and developed a module to compute the profiles of molecular abundances, taking the disequilibrium effect into account. Then, using ARCiS updated with this module, we performed retrievals of the observed transmission spectra of 16 exoplanets with sizes ranging from that of Jupiter to mini-Neptunes. Results. We find indications of disequilibrium chemistry for HD 209458b (≥4.1σ) and WASP-39b (≥2.7σ). The retrieved spectrum of HD 209458b exhibits a strong NH3 absorption feature at 10.5 μm that is accessible by JWST owing to an enhanced abundance of NH3 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 compared to the retrieval with the equilibrium assumption, such as a 2.9σ 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. Conclusions. We have demonstrated that the assumption of equilibrium chemistry can lead to a misinterpretation of the observed data, showing that spectral retrieval with a consideration of disequilibrium chemistry is essential in the era of JWST and Ariel.