Theoretical Transmission Spectra of Exoplanet Atmospheres with Hydrocarbon Haze. II. Dependence on UV Irradiation Intensity, Metallicity, C/O Ratio, Eddy Diffusion Coefficient, and Temperature

Abstract

Recent transmission spectroscopy has revealed that clouds and hazes are common in the atmospheres of close-in exoplanets. In this study, using the photochemical, microphysical, and transmission spectrum models for close-in warm ($\lesssim$ 1000 K) exoplanet atmospheres that we newly developed in our preceding paper (Kawashima & Ikoma 2018), we investigate the vertical distributions of haze particles and gaseous species and the resultant transmission spectra over wide ranges of the model parameters including UV irradiation intensity, metallicity, carbon-to-oxygen ratio (C/O), eddy diffusion coefficient, and temperature. The sensitivity to metallicity is of particular interest. We find that a rise in metallicity leads basically to reducing the photodissociation rates of the hydrocarbons and therefore the haze monomer production rates. This is due to an enhanced photon-shielding effect by the major photon absorbers such as ${\mathrm{H}}_2\mathrm{O}$, $\mathrm{CO}$, $\mathrm{C}{\mathrm{O}}_2$, and ${\mathrm{O}}_2$, existing at higher altitudes than the hydrocarbons. We also find that at relatively short wavelengths ($\lesssim$ 2-3 $\mu$m), the absorption features in transmission spectra are most pronounced for moderate metallicities such as 100 times the solar metallicity, whereas the lower the metallicity the stronger the absorption features at relatively long wavelengths ($\gtrsim$ 2-3 $\mu$m), where the contribution of haze is small. These are because of the two competing effects of reduced haze production rate and atmospheric scale height for higher metallicities. For the other model parameters, we show that stronger absorption features appear in transmission spectra of the atmospheres with lower UV irradiation, lower C/O ratio, higher eddy diffusion coefficient, and higher temperature.

28 pages, 24 figures, accepted for publication in ApJ, title shortened

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• The Hubble PanCET Program: A Featureless Transmission Spectrum for WASP-29b and Evidence of Enhanced Atmospheric Metallicity on WASP-80b
• Low-resolution Transit Spectroscopy of Three Hot Jupiters Using the 2m Himalayan Chandra Telescope