The Influence of H ₂ O Pressure Broadening in High-metallicity Exoplanet Atmospheres

Planet formation models suggest broad compositional diversity in the sub-Neptune/super-Earth regime, with a high likelihood for large atmospheric metal content (≥100× Solar). With this comes the prevalence of numerous plausible bulk atmospheric constituents including N ₂ , CO ₂ , H ₂ O, CO, and CH ₄ . 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 H ₂ /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 s of parts-per-million difference in the observed transmission and emission spectra and can significantly alter the one-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 the James Webb Space Telescope.