Intrinsic Lyman alpha Profiles of High-Velocity G, K, and M Dwarfs

TL;DR

This study uses high-resolution UV spectroscopy of five high-velocity G, K, and M dwarfs to analyze their intrinsic Lyman alpha emission profiles, revealing self-reversal features and implications for flux reconstruction accuracy.

Contribution

It provides the first direct observations of Lyman alpha line cores in high-velocity stars, showing self-reversal and limitations of Mg II as a template for flux reconstruction.

Findings

Self-reversal observed in all targets' Lyman alpha lines.

Self-reversal depth decreases with increasing surface gravity.

Neglecting self-reversal can lead to significant overestimation of intrinsic flux.

Abstract

Observations of H I Lyman alpha, the brightest UV emission line of late-type stars, are critical for understanding stellar chromospheres and transition regions, modeling photochemistry in exoplanet atmospheres, and measuring the abundances of neutral hydrogen and deuterium in the interstellar medium. Yet, Lyman alpha observations are notoriously challenging due to severe attenuation from interstellar gas, hindering our understanding of this important emission line's basic morphology. We present high-resolution far- and near-UV spectroscopy of five G, K, and M dwarfs with radial velocities large enough to Doppler shift the stellar Lyman alpha emission line away from much of the interstellar attenuation, allowing the line core to be directly observed. We detect self-reversal in the Lyman alpha emission line core for all targets, and we show that the self-reversal depth decreases with increasing surface gravity. Mg II self-reversed emission line profiles provide some useful information to constrain the Lyman alpha line core, but the differences are significant enough that Mg II cannot be used directly as an intrinsic Lyman alpha template during reconstructions. We show that reconstructions that neglect self-reversal could overestimate intrinsic Lyman alpha fluxes by as much as 60%-100% for G and K dwarfs and 40%-170% for M dwarfs. The five stars of our sample have low magnetic activity and sub-solar metallicity; a larger sample size is needed to determine how sensitive these results are to these factors.