Aerosol Constraints on the Atmosphere of the Hot Saturn-mass planet WASP-49b

TL;DR

This study demonstrates how to characterize aerosol layer boundaries and composition in exoplanet atmospheres with featureless spectra, using a combination of hydrodynamic and radiative transfer models applied to WASP-49b.

Contribution

It introduces a method to constrain aerosol layer pressures and compositions in exoplanet atmospheres despite featureless spectra, using Bayesian modeling and pressure-temperature analysis.

Findings

Aerosol layers are constrained between 10^{-5} and 10^{-7} bar.

Possible aerosol compositions include Na₂S, Cr, MnS, and silicates.

Method enables atmospheric characterization even with flat transmission spectra.

Abstract

The strong, nearly wavelength-independent absorption cross section of aerosols produces featureless exoplanet transmission spectra, limiting our ability to characterize their atmospheres. Here we show that even in the presence of featureless spectra, we can still characterize certain atmospheric properties. Specifically, we constrain the upper and lower pressure boundaries of aerosol layers, and present plausible composition candidates. We study the case of the bloated Saturn-mass planet WASP-49b, where near-infrared observations reveal a flat transmission spectrum between 0.7 and 1.0 {\microns}. First, we use a hydrodynamic upper-atmosphere code to estimate the pressure reached by the ionizing stellar high-energy photons at $10^{-8}$ bar, setting the upper pressure boundary where aerosols could exist. Then, we combine HELIOS and Pyrat Bay radiative-transfer models to constrain the temperature and photospheric pressure of atmospheric aerosols, in a Bayesian framework. For WASP-49b, we constrain the transmission photosphere (hence, the aerosol deck boundaries) to pressures above $10^{-5}$ bar (100$\times$ solar metallicity), $10^{-4}$ bar (solar), and $10^{-3}$ bar (0.1$\times$ solar) as lower boundary, and below $10^{-7}$ bar as upper boundary. Lastly, we compare condensation curves of aerosol compounds with the planet's pressure-temperature profile to identify plausible condensates responsible for the absorption. Under these circumstances, we find as candidates: Na$_{2}$S (at 100$\times$ solar metallicity); Cr and MnS (at solar and 0.1$\times$ solar); and forsterite, enstatite, and alabandite (at 0.1$\times$ solar).