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

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 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/heliumdominated atmospheres, namely CH 4 , CO, H 2 O, NH 3 , N 2 , and CO 2 , 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 have performed retrievals of the observed transmission spectra of 16 exoplanets with sizes ranging from Jupiter to mini-Neptune. 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 NH 3 absorption feature at 10.5 µm accessible by JWST owing to an enhanced abundance of 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 2.9σ 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.

Measuring Elemental Abundances of JWST Target Stars for Exoplanet Characterization I. FGK Stars

“…A recent paper by Kawashima & Min (2021) provides planetary C/O and N/O for 16 giant planets based on the spectral atmospheric retrieval program ARCiS (Min et al 2020). This program uses spectral data to solve for the atmospheric parameters of a planet using disequilibrium chemistry.…”

“…This program uses spectral data to solve for the atmospheric parameters of a planet using disequilibrium chemistry. Five of the planets analyzed by Kawashima & Min (2021) orbit stars analyzed by this work, which allows us to compare our stellar elemental ratios with those computed for the planets.…”

“…The compiled information for NLTE corrections here will inform future studies on elemental abundances. Finally, in Section 6.2 we applied our results with the planetary results of Kawashima & Min (2021) to provide preliminary information on the formation pathways of 5 giant planets in our sample.…”

“…Elemental ratios as defined byTurrini et al (2021) for stars analyzed by this paper and with planets inKawashima & Min (2021).…”

A physically derived eddy parametrization for giant planet atmospheres with application on hot-Jupiters

“…The use of a 1D model can overcome these limitations, however, it cannot reproduce fundamentally 3D processes like atmospheric dynamics. Nevertheless, when working on disequilibrium chemistry and haze microphysics via a 1D model, vertical atmospheric mixing has to be taken into account (Spiegel et al 2009;Moses et al 2011;Lavvas et al 2014;Lavvas & Koskinen 2017;Wong et al 2020;Gao et al 2020;Kawashima & Min 2021). This vertical transport results from multiple phenomena at different scales such as convection (Gierasch & Conrath 1985;Ackerman & Marley 2001), turbulent flow arising from horizontal shear stress (Holton 2004;Steinrueck et al 2020), and gravity waves (Lindzen 1971;Hunten 1975;Chamberlain 1978).…”