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.