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Astrophysics Luncheon Seminar

Theoretical Modeling of Transmission Spectra of Exoplanet Atmospheres with Hydrocarbon Haze and Applications to Multi-wavelength Transit Observations
Presented by Yui Kawashima
Earth-Life Science Institute, Tokyo Institute of Technology

Thursday, August 9, 2018
11:00 A.M. in 169-336

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 with 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.

JPL Contact: Renyu Hu (4-6090)


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