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Register a new userTheoretical Transmission Spectra of Exoplanet Atmospheres with Hydrocarbon Haze. II. Dependence on UV Irradiation Intensity, Metallicity, C/O Ratio, Eddy Diffusion Coefficient, and Temperature
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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 $mathrm{H_2O}$, $mathrm{CO}$, $mathrm{CO_2}$, 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.
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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 infer 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 used some assumed distribution and size of haze particles. In this study, we model the atmospheric chemistry, derive the spatial and size distributions of haze particles by simulating the creation, growth and settling of hydrocarbon haze particles directly, and develop transmission spectrum models of UV-irradiated, solar-abundance atmospheres of close-in warm ($sim$ 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.
Much of the focus of exoplanet atmosphere analysis in the coming decade will be atinfrared wavelengths, with the planned launches of the James Webb Space Telescope (JWST) and the Wide-Field Infrared Survey Telescope (WFIRST). However, without being placed in the context of broader wavelength coverage, especially in the optical and ultraviolet, infrared observations produce an incomplete picture of exoplanet atmospheres. Scattering information encoded in blue optical and near-UV observations can help determine whether muted spectral features observed in the infrared are due to a hazy/cloudy atmosphere, or a clear atmosphere with a higher mean molecular weight. UV observations can identify atmospheric escape and mass loss from exoplanet atmospheres, providing a greater understanding of the atmospheric evolution of exoplanets, along with composition information from above the cloud deck. In this white paper we focus on the science case for exoplanet observations in the near-UV; an accompanying white paper led by Eric Lopez will focus on the science case in the far-UV.
New observing capabilities coming online over the next few years will provide opportunities for characterization of exoplanet atmospheres. However, clouds/hazes could be present in the atmospheres of many exoplanets, muting the amplitude of spectral features. We use laboratory simulations to explore photochemical haze formation in H2-rich exoplanet atmospheres at 800 K with metallicity either 100 and 1000 times solar. We find that haze particles are produced in both simulated atmospheres with small particle size (20 to 140 nm) and relative low production rate (2.4 x 10-5 to 9.7 x 10-5 mg cm-3 h-1), but the particle size and production rate is dependent on the initial gas mixtures and the energy sources used in the simulation experiments. The gas phase mass spectra show that complex chemical processes happen in these atmospheres and generate new gas products that can further react to form larger molecules and solid haze particles. Two H2-rich atmospheres with similar C/O ratios (~0.5) yield different haze particles size, haze production rate, and gas products, suggesting both the elemental abundances and their bonding environments in an atmosphere can significantly affect the photochemistry. There is no methane (CH4) in our initial gas mixtures, although CH4 is often believed to be required to generate organic hazes. However, haze production rates from our experiments with different initial gas mixtures indicate that CH4 is neither required to generate organic hazes nor necessary to promote the organic haze formation. The variety and relative yield of the gas products indicate that CO and N2 enrich chemical reactions in H2-rich atmospheres.
Exoplanets on eccentric orbits experience an incident stellar flux that can be markedly larger at periastron versus apoastron. This variation in instellation can lead to dramatic changes in atmospheric structure in regions of the atmosphere where the radiative and advective heating/cooling timescales are shorter than the orbital timescale. To explore this phenomenon, we develop a sophisticated one-dimensional (vertical) time-stepping atmospheric structure code, EGP+, capable of simulating the dynamic response of atmospheric thermal and chemical structure to time-dependent perturbations. Critically, EGP+ can efficiently simulate multiple orbits of a planet, thereby providing new opportunities for exoplanet modeling without the need for more computationally-expensive models. We make the simplifying assumption of cloud-free atmospheres, and apply our model to HAT-P-2b, HD~17156b, and HD~80606b, which are known to be on higher-eccentricity orbits. We find that for those planets which have Spitzer observations, our planet-to-star ratio predictions are roughly consistent with observations. However, we are unable to reproduce the observed peak offsets from periastron passage. Finally, we discuss promising pathways forward for adding new model complexity that would enable more detailed studies of clear and cloudy eccentric planets as well as worlds orbiting active host stars.
Some of the exoplanets so far observed show featureless or flat transmission spectra, possibly indicating the existence of clouds and/or haze in their atmospheres. Thanks to its large aperture size and broad wavelength coverage, JWST is expected to enable detailed investigation of exoplanet atmospheres, which could provide important constraints on the atmospheric composition obscured by clouds/haze. Here, we use four warm ($lesssim 1000$ K) planets suitable for atmospheric characterization via transmission spectroscopy, GJ 1214b, GJ 436b, HD 97658b, and Kepler-51b, as examples to explore molecular absorption features detectable by JWST even in the existence of hydrocarbon haze in the atmospheres. We simulate photochemistry, the growth of hydrocarbon haze particles, and transmission spectra for the atmospheres of these four planets. Among the planetary parameters considered, super-Earths with hazy, relatively hydrogen-rich atmospheres are mostly expected to produce detectable molecular absorption features such as a quite prominent $mathrm{CH_4}$ feature at 3.3 ${rm mu}$m even for the extreme case of the most efficient production of photochemical haze. For a planet that has extremely low gravity, such as Kepler-51b, haze particles grow significantly large in the upper atmosphere due to the small sedimentation velocity, resulting in the featureless or flat transmission spectrum in a wide wavelength range. This investigation shows that the transmission spectra with muted features measured by HST in most cases do not preclude strong features at the longer wavelengths accessible by JWST.
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