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A Cloudy Fit to the Atmosphere of WASP-107 b

Helong Huang, Michiel Min, Chris W. Ormel, Achrène Dyrek, Nicolas Crouzet

Abstract

Context. WASP-107 b has been observed comprehensively by JWST in the near- and mid-IR bands, making it an ideal planet to probe the composition and internal dynamics. Recent analysis reveals a 8-10 um silicate feature, but it still remains uncertain how silicate clouds form on this planet. Aims. We aim at fitting the complete JWST spectrum of WASP-107 b, from 0.9 um to 12 um with a physically motivated cloud model and self-consistent temperature profile. Methods. Two-stream radiative transfer is coupled to a cloud formation model until convergence between cloud and temperature profiles is reached. We search a model grid spanning metallicity, turbulent diffusivity, internal heat flux and nucleation parameters to find the best fit model. Results. The silicate cloud feature at 10 um and the near-IR molecular band strength can be simultaneously and naturally explained without assuming a parametrized temperature profile. A moderate vertical diffusivity of Kzz = 10^9 cm^2 s^-1 is needed to bring the cloud particles to the upper atmosphere of WASP-107 b. This Kzz is favored by the joint fitting of the near-IR water feature and mid-IR silicate feature -- both sensitive to clouds. From the strength of H2O and CO2 bands, our model suggests a metallicity 17 times solar. Conclusions. Even in warm planets such as WASP-107 b, silicate clouds can form in the relatively cool upper atmosphere because turbulence uplifts vapor and cloud particles. Despite having considerably fewer degrees of freedom, the self-consistent modeling approach successfully fits WASP-107 b's multi-wavelength data, instilling confidence in the derived physical parameters.

A Cloudy Fit to the Atmosphere of WASP-107 b

Abstract

Context. WASP-107 b has been observed comprehensively by JWST in the near- and mid-IR bands, making it an ideal planet to probe the composition and internal dynamics. Recent analysis reveals a 8-10 um silicate feature, but it still remains uncertain how silicate clouds form on this planet. Aims. We aim at fitting the complete JWST spectrum of WASP-107 b, from 0.9 um to 12 um with a physically motivated cloud model and self-consistent temperature profile. Methods. Two-stream radiative transfer is coupled to a cloud formation model until convergence between cloud and temperature profiles is reached. We search a model grid spanning metallicity, turbulent diffusivity, internal heat flux and nucleation parameters to find the best fit model. Results. The silicate cloud feature at 10 um and the near-IR molecular band strength can be simultaneously and naturally explained without assuming a parametrized temperature profile. A moderate vertical diffusivity of Kzz = 10^9 cm^2 s^-1 is needed to bring the cloud particles to the upper atmosphere of WASP-107 b. This Kzz is favored by the joint fitting of the near-IR water feature and mid-IR silicate feature -- both sensitive to clouds. From the strength of H2O and CO2 bands, our model suggests a metallicity 17 times solar. Conclusions. Even in warm planets such as WASP-107 b, silicate clouds can form in the relatively cool upper atmosphere because turbulence uplifts vapor and cloud particles. Despite having considerably fewer degrees of freedom, the self-consistent modeling approach successfully fits WASP-107 b's multi-wavelength data, instilling confidence in the derived physical parameters.
Paper Structure (7 sections, 2 equations, 4 figures, 2 tables)

This paper contains 7 sections, 2 equations, 4 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Results
  3. Conclusions and Discussion
  4. Methods
  5. Coupling between cloud formation and radiative transfer
  6. Adjustment to the equilibrium chemistry
  7. Higher metallicity simulations

Figures (4)

  • Figure 1: Self-consistent temperature and cloud profiles, resulting from the joint cloud and radiative transfer model. Left: cloud profiles obtained for three $K_{zz}$ values. The best fit model has $K_{zz}=10^{9}\ \mathrm{cm}^2\mathrm{s}^{-1}$ (the line with golden highlight) and the corresponding particle size is shown by the purple line. The horizontal gray lines indicates the visible and IR photosphere. Right: cloud composition by mass fraction of the best-fit model. The gray line shows the atmosphere temperature profile of the best fit.
  • Figure 2: JWST observations of WASP-107 b (colored vertical bars) along with the best fit model (black line; $K_\mathrm{zz}=10^{9}\ \mathrm{cm}^2\mathrm{s}^{-1}$), resulting in a $\tilde{\chi}^2_\mathrm{red}=3.4$. The NIRISS observations are shifted vertically by $130$ ppm to match the NIRCam data at the wavelengths where they overlap. The runs with higher and lower $K_{zz}$ are shown with blue and brown lines, respectively, to illustrate the spectroscopic effects of varying the turbulent diffusivity. The simulated spectrum of a cloudless atmosphere is plotted in gray.
  • Figure 3: Scatter plots demonstrating how the near-IR H2O line strength and the $10\ \mathrm{\mu m}$ silicate metric depend on the diffusivity parameter ($K_{zz}$; left), metallicity (Z; middle) and internal energy flux ($T_\mathrm{int}$; right) model parameters. The size of the dots corresponds to the $\tilde{\chi}^2$ of their fits to the JWST observations. The gray rectangle corresponds to the JWST observational data and their $1\sigma$ uncertainties. The best-fit model run (star), is in $2\sigma$ agreement with the observation.
  • Figure 4: Same as Figure \ref{['fig:spectrum']}, but with metallicity $Z=56\ Z_\odot$ and $Z=3.2\ Z_\odot$. For each metallicity, the control parameters (Appendix \ref{['sec:diseq-adjust']}) are kept the same as the best-fit model.