Detectable Abundance of Cyanoacetylene (HC 3 N) Predicted on Reduced Nitrogen-rich Super-Earth Atmospheres

We predict that cyanoacetylene (HC3N) 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 (N2), methane (CH4), and hydrogen cyanide (HCN), as described by Swain et al. First, we construct line lists and cross sections for HC3N. Then we apply these cross sections and the model atmosphere of Swain et al. 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 μm, observable by JWST after one transit. This feature persists within the 1σ uncertainty of the retrieved abundances of HCN and CH4. The signal is detectable for stratospheric temperatures ≲600 K and moderate stratospheric mixing (106 cm2 s−1 ≲ Kzz ≲ 108 cm2 s−1). Our results also indicate that HC3N is an important source of opacity that future retrieval models should consider.