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

TL;DR

This study shows that nitrogen-rich atmospheres on hot super-Earths can develop strong temperature inversions due to CN absorption, significantly affecting their emission spectra and offering insights into their atmospheric chemistry.

Contribution

It demonstrates the potential for thermal inversions caused by CN in nitrogen-rich, high-temperature atmospheres of super-Earths, expanding understanding of their atmospheric structure.

Findings

CN causes strong temperature inversions in atmospheres above 2000 K

Inverted atmospheres show inverted absorption features in emission spectra

Hot super-Earths are ideal targets for JWST and ARIEL observations

Abstract

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 ultra-short-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 micron. Our results indicate that dueto shortwave absorption of CN, atmospheres with temperatures above 2000 K and C/O $\geq$ 1.0 are prone to thermal inversions. CN is one of the few molecules that is extremely stable at large temperatures occurring on the day side 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.