Effect of Pressure Broadening on Emission and Transmission Spectra of H2O Modeled for sub-Neptune/super-Earth exoplanets: An Application to JWST
Abstract
Water is the most readily detected molecule over a diverse range of exoplanet properties (solar composition hot-Jupiters to high metallicity super-earths/neptunes). It is also one of the most important sources of opacity that govern radiative energy balance. It is well known that pressure/collisional broadening significantly influences the opacity of a given molecule. Laboratory spectroscopic studies have shown that the line-broadening (i.e. Doppler, Lorentzian) is influence by several factors including temperature, pressure, dipole moment of the broadeners (or bath gases), and the rotational quantum numbers. Since absorption cross-sections (or opacities) are central to both atmospheric modeling and observational research, there is a critical need to investigate the effect of pressure line-broadening on the absorption cross-sections and subsequent influence on observed transmission and emission spectra of transiting exoplanets. Typical data-model comparisons (either forward modeling or retrieval's) generally rely upon pre-computed grids of absorption cross-sections that assume trace molecules are broadened by a solar composition mixture (e.g., mainly H2 and He as a bath gas). However, as the metallicity of a planetary atmosphere increases, as anticipated for smaller planets in the sub-Neptune-Super Earth range, broadening due to other gases (e.g., N2, CO2, H2O, CH4, CO) can become significant and the H2-He broadening is no longer appropriate. In this work, we assess the influence of different background broadeners on the absorption cross-section of water, and subsequent influence on observed transmission/emission spectra. Initial results suggest that the choice of foreign broadener can result in up to a factor of ~5 increase in pressure broadened wings of the absorption cross sections, resulting in a factor of 1.6x reduction in the atmospheric spectral modulation. Such a difference will be detectible in the hi-resolution/SNR spectra anticipated with JWST, and will certainly influence the interpretation of high-metallicity atmospheres.
- Publication:
-
AAS/Division for Planetary Sciences Meeting Abstracts #49
- Pub Date:
- October 2017
- Bibcode:
- 2017DPS....4941619G