Abstract
Context.Clouds are ubiquitous in exoplanet atmospheres and they represent a challenge for the model interpretation of their spectra. When generating a large number of model spectra, complex cloud models often prove too costly numerically, whereas more efficient models may be overly simplified.Aims.We aim to constrain the atmospheric properties of the directly imaged planet HR 8799e with a free retrieval approach.Methods.We used our radiative transfer code petitRADTRANS for generating the spectra, which we coupled to the PyMultiNest tool. We added the effect of multiple scattering which is important for treating clouds. Two cloud model parameterizations are tested: the first incorporates the mixing and settling of condensates, the second simply parameterizes the functional form of the opacity.Results.In mock retrievals, using an inadequate cloud model may result in atmospheres that are more isothermal and less cloudy than the input. Applying our framework on observations of HR 8799e made with the GPI, SPHERE, and GRAVITY, we find a cloudy atmosphere governed by disequilibrium chemistry, confirming previous analyses. We retrieve that C/O = 0.60−0.08+0.07. Other models have not yet produced a well constrained C/O value for this planet. The retrieved C/O values of both cloud models are consistent, while leading to different atmospheric structures: either cloudy or more isothermal and less cloudy. Fitting the observations with the self-consistent Exo-REM model leads to comparable results, without constraining C/O.Conclusions.With data from the most sensitive instruments, retrieval analyses of directly imaged planets are possible. The inferred C/O ratio of HR 8799e is independent of the cloud model and thus appears to be a robust. This C/O is consistent with stellar, which could indicate that the HR 8799e formed outside the CO2or CO iceline. As it is the innermost planet of the system, this constraint could apply to all HR 8799 planets.
Highlights
The description of clouds in exoplanets and brown dwarfs is among the major uncertainties when modeling the structures and spectra of self-luminous atmospheres (e.g., Marley et al 2013)
Focusing on the bulk atmospheric properties (log (g), carbon-to-oxygen number ratio (C/O), [Fe/H]), we find that the cloudy mode of the retrieval with Cloud Model 2 is fully consistent with the results from our nominal retrieval with Cloud Model 1
As we find that the planet has a C/O ratio consistent with its host star
Summary
The description of clouds in exoplanets and brown dwarfs is among the major uncertainties when modeling the structures and spectra of self-luminous atmospheres (e.g., Marley et al 2013). Some of the “free parameters” of such elaborate cloud models are likely to not be free at all, but are determined by the full solution of the (multi-dimensional) atmospheric structure, which is a function of the cloud properties itself due to the radiative feedback of the clouds. At the same time, such complicated cloud models are useful and necessary because they allow us to understand the interplay of physical processes during cloud formation and will have to explain the cloud properties of all exoplanets and brown dwarfs, whether irradiated or self-luminous.
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