2024MNRAS.530..482A

We present a study on the coupling of haze and clouds in the atmosphere of WASP-39b. We developed a cloud microphysics model simulating the formation of Na₂S and MgSiO₃ condensates over photochemical hazes in gas giant atmospheres. We apply this model to WASP-39b, recently observed with the JWST to study how these heterogeneous components may affect the transit spectrum. We simulate both morning and evening terminators independently and average their transit spectra. While MgSiO₃ formation has negligible impact on the spectrum, Na₂S condensates produce grey opacities in the water band, in agreement with Hubble Space Telescope (HST) and JWST observations. Moreover, the formation of Na₂S on the morning side depletes the atmosphere of its sodium content, decreasing the strength of the Na line. Combining morning and evening profiles results in a good fit of the Na observations. These nominal results assume a small Na₂S/haze contact angle (5.7°). Using a larger value (61°) reduces the cloud density and opacity, but the effect on the Na profile and spectral line remains identical. In addition, the presence of haze in the upper atmosphere reproduces the ultraviolet (UV)-visible slope observed in the HST and Very Large Telescope data and contributes to the opacity between the water bands at wavelengths below 2 μm. The averaged spectra are rather insensitive to the variation of eddy diffusion and haze mass flux tested in this study, though the UV-visible slope, probing the haze layer above the clouds, is affected. Finally, our disequilibrium chemistry model, including photochemistry, reproduces the SO₂ and CO₂ absorption features observed.