Temperature inversions on hot super-Earths: the case of CN in nitrogen-rich atmospheres

We show that in extremely irradiated atmospheres of hot super-Earths shortwave absorption of CN can cause strong temperature inversions. We base this study on previous observations of 55 Cancri e, which lead us to believe that ultrashort-period super-Earths can sustain volatile atmospheres, rich in nitrogen and/or carbon. We compute our model atmospheres in a radiative-convective equilibrium for a variety of nitrogen-rich cases and orbital parameters. We demonstrate the effects caused by thermal inversions on the chemistry and compute low-resolution synthetic emission spectra for a range of 0.5-28 $\rm{\mu m}$ . Our results indicate that due to shortwave absorption of CN, atmospheres with temperatures above 2000 K and C/O ≥ 1.0 are prone to thermal inversions. CN is one of the few molecules that is extremely stable at large temperatures occurring on the dayside of short-period super-Earths. The emission spectrum of such atmospheres will differ substantially from non-inverted cases. In the case of inversions, absorption features become inverted, showing higher than expected flux. We propose that inversions in hot atmospheres should be the expected norm. Hot super-Earths are some of the most extreme natural laboratories for testing predictions of atmospheric chemistry and structure. They are frequently occurring, bright in emission and have short orbital periods. All these factors make them perfect candidates to be observed with JWST and ARIEL missions.