Optical Transmission Spectra of Hot Jupiters: Effects of Scattering

Abstract

We present new grids of transmission spectra for hot Jupiters by solving the multiple-scattering radiative transfer equations with non-zero scattering albedo instead of using the Beer–Bouguer–Lambert law for the change in the transmitted stellar intensity. The diffused reflection and transmission due to scattering increase the transmitted stellar flux resulting, in a decrease in the transmission depth. Thus, we demonstrate that scattering plays a double role in determining the optical transmission spectra—increasing the total optical depth of the medium and adding the diffused radiation due to scattering to the transmitted stellar radiation. The resulting effects yield an increase in the transmitted flux and hence a reduction in the transmission depth. For a cloudless planetary atmosphere, Rayleigh scattering albedo alters the transmission depth up to about 0.6 μm, but the change in the transmission depth due to forward scattering by cloud or haze is significant throughout the optical and near-infrared regions. However, at wavelengths longer than about 1.2 μm, the scattering albedo becomes negligible, hence the transmission spectra match with that calculated without solving the radiative transfer equations. We compare our model spectra with existing theoretical models and find significant differences at wavelengths shorter than one micron. We also compare our models with observational data for a few hot Jupiters, which may help with constructing better retrieval models in the future.