Computing Intrinsic Lyman-alpha Fluxes of F5 V to M5 V Stars

TL;DR

This paper develops correlations to estimate the intrinsic Lyman-alpha flux of late-type stars using other observable stellar emissions, aiding exoplanet atmospheric studies where direct measurements are impossible.

Contribution

It introduces new correlations linking Lyman-alpha flux with other stellar emissions, enabling flux estimation for distant or faint stars without direct measurements.

Findings

Strong correlation between Lyman-alpha and X-ray flux for F5 V to K5 V stars.

Larger dispersion in Lyman-alpha correlation for M stars.

Effective temperature and rotation rate help estimate Lyman-alpha flux for G and K stars.

Abstract

The Lyman-alpha emission line dominates the far-ultraviolet spectra of late-type stars and is a major source for photodissociation of important molecules including H2O, CH4, and CO2 in exoplanet atmospheres. The incident flux in this line illuminating an exoplanet's atmosphere cannot be measured directly as neutral hydrogen in the interstellar medium (ISM) attenuates most of the flux reaching the Earth. Reconstruction of the intrinsic Lyman-alpha line has been accomplished for a limited number of nearby stars, but is not feasible for distant or faint host stars. We identify correlations connecting the intrinsic Lyman-alpha flux with the flux in other emission lines formed in the stellar chromosphere, and find that these correlations depend only gradually on the flux in the other lines. These correlations, which are based on HST spectra, reconstructed Lyman-alpha line fluxes, and irradiance spectra of the quiet and active Sun, are required for photochemical models of exoplanet atmospheres when intrinsic Lyman-alpha fluxes are not available. We find a tight correlation of the intrinsic Lyman-alpha flux with stellar X-ray flux for F5 V to K5 V stars, but much larger dispersion for M stars. We also show that knowledge of the stellar effective temperature and rotation rate can provide reasonably accurate estimates of the Lyman-alpha flux for G and K stars, and less accurate estimates for cooler stars.