TRANSMISSION SPECTRA OF TRANSITING PLANET ATMOSPHERES: MODEL VALIDATION AND SIMULATIONS OF THE HOT NEPTUNE GJ 436b FOR THE<i>JAMES WEBB SPACE TELESCOPE</i>

Abstract

We explore the transmission spectrum of the Neptune-class exoplanet GJ 436b,\nincluding the possibility that its atmospheric opacity is dominated by a\nvariety of non- equilibrium chemical products. We also validate our\ntransmission code by demonstrating close agreement with analytic models that\nuse only Rayleigh scattering or water vapor opacity. We find broad disagreement\nwith radius variations predicted by another published model. For GJ 436b, the\nrelative coolness of the planet's atmosphere, along with its implied high\nmetallicity, may make it dissimilar in character compared to "hot Jupiters."\nSome recent observational and modeling efforts suggest low relative abundances\nof H2O and CH4 present in GJ 436b's atmosphere, compared to calculations from\nequilibrium chemistry. We include these characteristics in our models and\nexamine the effects of absorption from methane-derived higher order\nhydrocarbons. Significant absorption from HCN and C2H2 are found throughout the\ninfrared, while C2H4 and C2H6 are less easily seen. We perform detailed\nsimulations of JWST observations, including all likely noise sources, and find\nthat we will be able to constrain chemical abundance regimes from this planet's\ntransmission spectrum. For instance, the width of the features at 1.5, 3.3, and\n7 microns indicates the amount of HCN versus C2H2 present. The NIRSpec prism\nmode will be useful due to its large spectral range and the relatively large\nnumber of photo-electrons recorded per spectral resolution element. However,\nextremely bright host stars like GJ 436 may be better observed with a higher\nspectroscopic resolution mode in order to avoid detector saturation. We find\nthat observations with the MIRI low resolution spectrograph should also have\nhigh signal-to-noise in the 5 - 10 micron range due to the brightness of the\nstar and the relatively low spectral resolution (R ~ 100) of this mode.\n