2018ApJ...853....7K

Recently, properties of exoplanet atmospheres have been constrained via multi-wavelength transit observation, which measures an apparent decrease in stellar brightness during planetary transit in front of its host star (called transit depth). Sets of transit depths so far measured at different wavelengths (called transmission spectra) are somewhat diverse: some show steep spectral slope features in the visible, some contain featureless spectra in the near-infrared, some show distinct features from radiative absorption by gaseous species. These facts imply the existence of haze in the atmospheres, especially of warm, relatively low-density super-Earths and mini-Neptunes. Previous studies that addressed theoretical modeling of transmission spectra of hydrogen-dominated atmospheres with haze made some assumptions about the distribution and size of haze particles. In this study, we model the atmospheric chemistry, and derive the spatial and size distributions of haze particles by directly simulating the creation, growth, and settling of hydrocarbon haze particles. We then develop transmission spectrum models of UV-irradiated, solar-abundance atmospheres of close-in warm (∼500 K) exoplanets. We find that the haze is distributed in the atmosphere much more broadly than previously assumed, and consists of particles of various sizes. We also demonstrate that the observed diversity of transmission spectra can be explained by the difference in the production rate of haze monomers, which is related to the UV irradiation intensity from host stars.