Reliability of 1D radiative-convective photochemical-equilibrium retrievals on transit spectra of WASP-107b

Abstract

Observations of WASP-107b suggest a metal-rich and carbon-deprived atmosphere with an extremely hot interior based on detections of SO${}_2$, H${}_2$O, CO${}_2$, CO, NH${}_3$, and CH${}_4$. In this paper, we aim to determine the reliability of a 1D radiative-convective photochemical-equilibrium (1D-RCPE) retrieval method in inferring atmospheric properties of WASP-107b. Our grid of radiative-convective balanced pressure-temperature profiles and 1D photochemical equilibrated models covers a range of metallicities (Z), carbon-to-oxygen ratios (C/O), intrinsic temperatures (T${}_{int}$), and eddy diffusion coefficients (K${}_{zz}$). We obtain good fits with our 1D-RCPE retrievals based on a few molecular features of H${}_2$O, CO${}_2$, SO${}_2$, and CH${}_4$, but find no substantial contribution of NH${}_3$. We find that the degeneracy between metallicity, cloud pressure, and a model offset is broken by the presence of strong SO${}_2$ features, confirming that SO${}_2$ is a robust metallicity indicator. We systematically retrieve sub-solar C/O based on the relative amplitude of a strong CO${}_2$ feature with respect to the broad band of H${}_2$O, which is sensitive to a wavelength-dependent scattering slope. We find that high-altitude clouds obscure the CH${}_4$-rich layers, preventing the retrievals from constraining T${}_{int}$, but that higher values of K${}_{zz}$ can transport material above the cloud deck, allowing a fit of the CH${}_4$ feature. However, T${}_{int}$ and K${}_{zz}$ can vary substantially between retrievals depending on the adopted cloud parametrization. We conclude that the 1D-RCPE retrieval method can provide useful insights if the underlying grid of forward models is well understood. We find that WASP-107b's atmosphere is enriched in metals (3 to 5 times solar) and carbon-deprived (C/O <= 0.20). However, we lack robust constraints on the intrinsic temperature and vertical mixing strength.

30 pages, 18 figures, 5 tables, accepted by A&A

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