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Coupling haze and cloud microphysics in WASP-39b’s atmosphere based on JWST observations

Presentation #200.08 in the session Exoplanet Atmospheres: Giant Planets (Oral Presentation)

Published onOct 23, 2023
Coupling haze and cloud microphysics in WASP-39b’s atmosphere based on JWST observations

Haze and clouds are expected in many exoplanet atmospheres with various composition. Here, we focus on the hot-Jupiter WASP-39b, which has been observed with the JWST (Ahrer et al., 2023a,b; Alderson et al., 2023; Feinstein et al., 2023; Rustamkulov et al., 2023). These transit observations provide further constrains on this planet’s properties, especially its metallicity and C/O ratio, and demonstrate that cloud opacities are expected to partially mute the water band. The 10×solar metallicity retrieved from the JWST observations is larger than the solar values typically assumed for hot-Jupiters (e.g. Arfaux & Lavvas, 2022) and has major ramifications for the haze and cloud properties. Indeed, while the presence of haze in WASP-39b atmosphere was considered unnecessary for reproducing the UV-Visible transit spectrum for a solar metallicity assumption, higher metallicity results in a weaker UV transit depth, therefore requires a high altitude absorber. Preliminary results on the inclusion of clouds indicate that both haze and clouds are required to fit the JWST data (Arfaux & Lavvas, 2023).

Most studies on cloud microphysics consider the heterogeneous nucleation of the condensing material over TiO2 particles formed through homogeneous nucleation. However, TiO2 is a high temperature condensate and is not expected to form in sufficient abundances in WASP-39b atmosphere (Carone et al., 2023). In this work, we propose that photochemical haze particles, already expected in WASP-39b atmosphere, can serve as nucleation sites, as they settle through the atmosphere.

We use a cloud and haze microphysics model to compute the size distributions of hazes and Na2S and MgSiO3 condensates, coupled to a disequilibrium chemistry model, thus allowing to study the effect of cloud formation on the chemical composition. We run individual simulations for the morning and evening terminators, using temperature profiles from Tsai et al. (2023). We find that Na2S and MgSiO3 clouds form in the morning terminator, while the relatively higher temperature of the evening terminator allows only for MgSiO3 cloud formation. We additionally test for the effect of changing the strength of the eddy diffusion and the mass flux of haze particles.

The combined gas, haze & cloud compositions of the two terminators provide a good fit to the JWST observations through out the spectrum, demonstrating the need for both clouds and hazes. Moreover, the depletion of Na on the morning terminator due to the Na2S cloud formation is instrumental for understanding the Na line observations.

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