The Influence of H2O Pressure Broadening in High Metallicity Exoplanet Atmospheres

TL;DR

This paper investigates how water self-broadening affects the spectra and temperature profiles of high-metallicity exoplanet atmospheres, emphasizing its importance for accurate interpretation of observational data.

Contribution

It demonstrates that water self-broadening significantly impacts spectral features and temperature structures in high-metallicity atmospheres, highlighting the need for correct broadening assumptions.

Findings

Water self-broadening causes 10-100 ppm differences in spectra.

Choice of broadener alters vertical temperature profiles.

Proper broadening models are essential for accurate exoplanet characterization.

Abstract

Planet formation models suggest broad compositional diversity in the sub-Neptune/super-Earth regime, with a high likelihood for large atmospheric metal content (> 100 x Solar). With this comes the prevalence of numerous plausible bulk atmospheric constituents including N2, CO2, H2O, CO, and CH4. Given this compositional diversity there is a critical need to investigate the influence of the background gas on the broadening of the molecular absorption cross-sections and the subsequent influence on observed spectra. This broadening can become significant and the common H2/He or "air" broadening assumptions are no longer appropriate. In this work we investigate the role of water self-broadening on the emission and transmission spectra as well as on the vertical energy balance in representative sub-Neptune/super-Earth atmospheres. We find that the choice of the broadener species can result in a 10 -- 100 parts-per-million difference in the observed transmission and emission spectra and can significantly alter the 1-dimensional vertical temperature structure of the atmosphere. Choosing the correct background broadener is critical to the proper modeling and interpretation of transit spectra observations in high metallicity regimes, especially in the era of higher precision telescopes such as JWST.