Lyman-$\alpha$ Observations of High Radial Velocity Low-Mass Stars Ross 1044 and Ross 825

TL;DR

This study uses high radial velocity observations from Hubble to better understand Lyman-alpha emissions in low-mass stars, improving models of their UV radiation environments relevant for habitable zone planets.

Contribution

It introduces new Lyman-alpha flux relations for low-mass stars based on HST observations of stars with high radial velocities, enhancing predictive models.

Findings

Established updated temperature-Lyman-alpha flux relation.

Developed model-independent UV-Lyman-alpha flux relationships.

Enabled better predictions of stellar UV environments for low-mass stars.

Abstract

The discovery of habitable zone (HZ) planets around low-mass stars has highlighted the need for a comprehensive understanding of the radiation environments in which such planets reside. Of particular importance is knowledge of the far-ultraviolet (FUV) radiation, as low-mass stars are typically much more active than solar-type stars and the proximity of their HZs can be one tenth the distance. The vast majority of the flux emitted by low-mass stars at FUV wavelengths occurs in the Lyman-$\alpha$ line at 1216 Angstroms. However, measuring a low-mass star's Lyman-$\alpha$ emission directly is almost always impossible because of the contaminating effects of interstellar hydrogen and geocoronal airglow. We observed Ross 825 (K3) and Ross 1044 (M0), two stars with exceptional radial velocities, with the STIS spectrograph aboard the Hubble Space Telescope (HST). Their radial velocities resulted in significant line shifts, allowing for a more complete view of their Lyman-$\alpha$ line profiles. We provide an updated relation between effective temperature and Lyman-$\alpha$ flux using Gaia DR2 astrometry as well as updated, model-independent relationships between Lyman-$\alpha$ flux and UV flux measurements from the Galaxy Evolution Explorer (GALEX) for low-mass stars. These new relations, in combination with GALEX's considerable spatial coverage, provide substantial predictive power for the Lyman-$\alpha$ environments for thousands of nearby, low-mass stars.