Theoretical transmission spectra of exoplanet atmospheres with hydrocarbon haze: Effect of creation, growth, and settling of haze particles. I. Model description and first results

摘要

Recently, properties of exoplanet atmospheres have been constrained viamulti-wavelength transit observation, which measures an apparent decrease instellar brightness during planetary transit in front of its host star (calledtransit depth). Sets of transit depths so far measured at different wavelengths(called transmission spectra) are somewhat diverse: Some show steep spectralslope features in the visible, some contain featureless spectra in thenear-infrared, some show distinct features from radiative absorption by gaseousspecies. These facts infer the existence of haze in the atmospheres especiallyof warm, relatively low-density super-Earths and mini-Neptunes. Previousstudies that addressed theoretical modeling of transmission spectra ofhydrogen-dominated atmospheres with haze used some assumed distribution andsize of haze particles. In this study, we model the atmospheric chemistry,derive the spatial and size distributions of haze particles by simulating thecreation, growth and settling of hydrocarbon haze particles directly, anddevelop transmission spectrum models of UV-irradiated, solar-abundanceatmospheres of close-in warm ($\sim$ 500 K) exoplanets. We find that the hazeis distributed in the atmosphere much more broadly than previously assumed andconsists of particles of various sizes. We also demonstrate that the observeddiversity of transmission spectra can be explained by the difference in theproduction rate of haze monomers, which is related to the UV irradiationintensity from host stars.