Cross-sections for heavy atmospheres: H$_2$O self-broadening

TL;DR

This study demonstrates that accounting for water self-broadening in atmospheric models significantly alters simulated transit depths for water-rich exoplanets, impacting future spectroscopic analyses with JWST and Ariel.

Contribution

It provides the first detailed analysis of the effects of water self-broadening on transmission spectra of super-Earths, highlighting the need for updated cross-sections in exoplanet atmospheric modeling.

Findings

Self-broadening causes up to 60 ppm increase in transit depth.

Differences are most significant in lighter, water-rich atmospheres.

Results are detectable with upcoming space telescopes JWST and Ariel.

Abstract

The discovery of super-Earth and mini-Neptune exoplanets means that atmospheric signals from low-mass, temperate exoplanets are being increasingly studied. The signal acquired as the planet transits its host star, known as the transit depth, is smaller for these planets and, as such, more difficult to analyze. The launch of the space telescopes James Webb (JWST) & Ariel will give rise to an explosion in the quality and quantity of spectroscopic data available for an unprecedented number of exoplanets in our galaxy. Accurately extracting the information content, thereby permitting atmospheric science, of such data-sets will require robust models and techniques. We present here the analysis of simulated transmission spectra for water-rich atmospheres, giving evidence for non-negligible differences in simulated transit depths when self-broadening of H$_2$O is correctly accounted for, compared with the currently typically accepted standard of using H$_2$ and He-broadened cross-sections. Our case-study analysis is carried out on two super-Earths, focusing on water-based atmospheres, ranging from H$_2$-rich to H$_2$O-rich. The transit depth is considerably affected, increasing values by up to 60 ppm, which is shown to be detectable with JWST and Ariel. The differences are most pronounced for the lighter (i.e. $mu$ $\sim$ 4) atmospheres. Our work illustrates that it is imperative that the field of exoplanet spectroscopy moves toward adapted cross-sections, increasingly optimized for high-mu atmospheres for studies of super-Earths and mini-Neptunes.