2021ApJ...911..112Y

The H I Lyα (1215.67 Å) emission line dominates the far-UV spectra of M dwarf stars, but strong absorption from neutral hydrogen in the interstellar medium makes observing Lyα challenging even for the closest stars. As part of the Far-Ultraviolet M-dwarf Evolution Survey, the Hubble Space Telescope has observed 10 early-to-mid M dwarfs with ages ranging from ∼24 Myr to several Gyr in order to evaluate how the incident UV radiation evolves through the lifetime of exoplanetary systems. We reconstruct the intrinsic Lyα profiles from STIS G140L and E140M spectra, and achieve reconstructed fluxes with 1σ uncertainties ranging from 5% to a factor of two for the low-resolution spectra (G140L) and 3%-20% for the high-resolution spectra (E140M). We observe broad, 500-1000 km s^–1 wings of the Lyα line profile, and analyze how the line width depends on stellar properties. We find that stellar effective temperature and surface gravity are the dominant factors influencing the line width with little impact from the star's magnetic activity level, and that the surface flux density of the Lyα wings may be used to estimate the chromospheric electron density. The Lyα reconstructions on the G140L spectra are the first attempted on λ/Δλ ∼ 1000 data. We find that the reconstruction precision is not correlated with the signal-to-noise ratio of the observation-rather, it depends on the intrinsic broadness of the stellar Lyα line. Young, low-gravity stars have the broadest lines and therefore provide more information at low spectral resolution to the fit to break degeneracies among model parameters.