Detectable Abundance of Cyanoacetylene (HC${}_3$N) Predicted on Reduced Nitrogen-Rich Super-Earth Atmospheres

Abstract

We predict that cyanoacetylene (HC${}_3$N) is produced photochemically in the atmosphere of GJ 1132 b in abundances detectable by the James Webb Space Telescope (JWST), assuming that the atmosphere is hydrogen dominated and rich in molecular nitrogen (N${}_2$), methane (CH${}_4$) and hydrogen cyanide (HCN), as described by Swain et al. (2021). First, we construct line list and cross-sections for HC${}_3$N. Then we apply these cross-sections and the model atmosphere of Swain et al. (2021) to a radiative transfer model in order to simulate the transmission spectrum of GJ 1132 b as it would be seen by JWST, accounting for the uncertainty in the retrieved abundances. We predict that cyanoacetylene features at various wavelengths, with a clear lone feature at 4.5 $\mu$m, observable by JWST after one transit. This feature persists within the $1-\sigma$ uncertainty of the retrieved abundances of HCN and CH${}_4$. The signal is detectable for stratospheric temperatures $\lesssim 600$ K and moderate stratospheric mixing ($10^6\mathrm{c}{\mathrm{m}}^2{\mathrm{s}}^{-1}\lesssim K_{zz}\lesssim 10^8\mathrm{c}{\mathrm{m}}^2{\mathrm{s}}^{-1}$). Our results also indicate that HC${}_3$N is an important source of opacity that future retrieval models should consider.

11 pages, 5 figures, accepted for publication in ApJL, comments welcome

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• The Effect of Metallicity on the Non-Equilibrium Abundance of Hydrogen Dominated Exoplanet Atmosphere