Accurate Modeling of Lyman-alpha Profiles and their Impact on Photolysis of Terrestrial Planet Atmospheres

TL;DR

This paper improves the modeling of Lyman-alpha emission lines from low-mass stars, crucial for understanding exoplanet atmospheres and photochemistry, by using high-velocity star observations to refine stellar spectra predictions.

Contribution

It introduces new observations of high-velocity stars to better model Lyman-alpha profiles and enhances the PHOENIX atmosphere code for more accurate stellar spectra predictions.

Findings

High-velocity star observations reveal intrinsic Lyα profiles.

Refined PHOENIX models better predict Lyα core emission.

Changes in stellar spectra significantly impact exoplanet atmospheric chemistry.

Abstract

Accurately measuring and modeling the Lyman-$\alpha$ (Ly$\alpha$; $\lambda$1215.67 \AA) emission line from low mass stars is vital for our ability to build predictive high energy stellar spectra, yet interstellar medium (ISM) absorption of this line typically prevents model-measurement comparisons. Ly$\alpha$ also controls the photodissociation of important molecules, like water and methane, in exoplanet atmospheres such that any photochemical models assessing potential biosignatures or atmospheric abundances require accurate Ly$\alpha$ host star flux estimates. Recent observations of three early M and K stars (K3, M0, M1) with exceptionally high radial velocities (>100 km s$^{-1}$) reveal the intrinsic profiles of these types of stars as most of their Ly$\alpha$ flux is shifted away from the geocoronal line core and contamination from the ISM. These observations indicate that previous stellar spectra computed with the PHOENIX atmosphere code have underpredicted the core of Ly$\alpha$ in these types of stars. With these observations, we have been able to better understand the microphysics in the upper atmosphere and improve the predictive capabilities of the PHOENIX atmosphere code. Since these wavelengths drive the photolysis of key molecular species, we also present results analyzing the impact of the resulting changes to the synthetic stellar spectra on observable chemistry in terrestrial planet atmospheres.