Title: Theoretical modeling of transmission spectra of exoplanet
atmospheres with hydrocarbon haze and applications to multi-wavelength
transit observations

Abstract:
A planet orbiting a star other than the Sun, which is often called an
exoplanet, was first discovered in 1995. Since then, detection of more than
3000 exoplanets has been reported. Recently, transit observations of an
exoplanet, which measures an apparent decrease in stellar brightness during
planetary transit in front of its host star (called transit depth), have
been done at multiple wavelengths. From the transit depth, we can measure
the planetary radius. In addition, observed dependence of the planetary
radius on wavelength (which is often called the transmission spectrum)
provides the information of absorption and scattering by molecules and
small particles such as haze in the planetary atmosphere. Thus, the
composition of the planetary atmosphere can be constrained by comparison
between the observational and theoretical transmission spectra. The
constraint on atmospheric composition gives an important clue to the origin
of the planet. Transmission spectra so far measured 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 infer the existence of
haze in the atmospheres.

Previous studies that addressed theoretical modeling of transmission
spectra of hydrogen-dominated atmospheres with haze used some assumed
distribution and size of haze particles and did not assess the viability of
those assumed haze properties sufficiently from a physical point of view.
In addition, although the previous studies found that various haze
parameters being chosen, one can generate the observed variation in
transmission spectra, it remains to be clarified what yields such a variety
of haze properties.

In this study, focusing on photochemically-produced hydrocarbon haze as a
possible candidate for the haze, we explore what diversity of transmission
spectra of exoplanets are brought from different production rates and
distributions of the monomers of haze particles, which are related to the
variety in UV intensities of current and near-future target stars for
exoplanet characterization, M dwarfs. To do so, we model the haze formation
processes, assuming hydrogen-dominated atmospheres of close-in warm (< 1000
K) exoplanets, derive the realistic distribution of the size and number
density of haze particles, and explore its impacts on transmission spectra.
Then, we explore the production rate of haze monomers and resultant
transmission spectra of the atmospheres of currently observable warm
exoplanets including GJ 1214b, GJ 3470b, and GJ 436b. As a result, we have
found that the haze particles tend to distribute more broadly in the
atmosphere than previously assumed and consist of various sizes. We have
also found that the difference in the production rate of haze monomers,
which relates to the UV irradiation intensity from the host star, yields
the diversity of transmission spectra observationally suggested.