Cross-sections for heavy atmospheres: H$_2$O continuum

TL;DR

This paper demonstrates that including water vapor continuum in atmospheric models significantly alters simulated exoplanet transit depths, especially for low-temperature, water-rich atmospheres, impacting future observational analyses.

Contribution

It provides the first detailed analysis of how water vapor continuum affects transmission spectra of temperate exoplanets, emphasizing the need for adaptive cross-sections in models.

Findings

Water vapor continuum can increase transit depth by up to 80 ppm.

Differences are detectable with future missions like JWST and Ariel.

Lower-temperature atmospheres are most affected by the continuum inclusion.

Abstract

Most of the exoplanets detected up to now transit in front of their host stars, allowing for the generation of transmission spectra; the study of exoplanet atmospheres relies heavily upon accurate analysis of these spectra. Recent discoveries mean that the study of atmospheric signals from low-mass, temperate worlds are becoming increasingly common. The observed transit depth in these planets is small and more difficult to analyze. Analysis of simulated transmission spectra for two small, temperate planets (GJ 1214 b and K2-18 b) is presented, giving evidence for significant differences in simulated transit depth when the water vapor continuum is accounted for when compared to models omitting it. These models use cross-sections from the CAVIAR lab experiment for the water self-continuum up to 10,000 cm$^{-1}$; these cross-sections exhibit an inverse relationship with temperature, hence lower-temperature atmospheres are the most significantly impacted. Including the water continuum strongly affects transit depths, increasing values by up to 80 ppm, with the differences for both planets being detectable with the future space missions Ariel and JWST. It is imperative that models of exoplanet spectra move toward adaptive cross-sections, increasingly optimized for H$_2$O-rich atmospheres. This necessitates including absorption contribution from the water vapor continuum into atmospheric simulations.