FUMES. II. Ly$\alpha$ Reconstructions of Young, Active M Dwarfs

TL;DR

This study reconstructs the intrinsic Lyα emission profiles of 10 young to old M dwarf stars using Hubble data, revealing how stellar properties influence line profiles and providing insights into stellar chromospheres and UV radiation relevant for exoplanet studies.

Contribution

First to successfully reconstruct Lyα profiles from low-resolution spectra of M dwarfs, analyzing how stellar parameters affect line width and flux, with implications for exoplanet habitability.

Findings

Stellar effective temperature and gravity dominate Lyα line width.

Reconstructed fluxes have uncertainties of 5-20%.

Surface flux density of Lyα wings can estimate chromospheric electron density.

Abstract

The HI Ly$\alpha$ (1215.67 $\unicode{xC5}$) emission line dominates the far-UV spectra of M dwarf stars, but strong absorption from neutral hydrogen in the interstellar medium makes observing Ly$\alpha$ challenging even for the closest stars. As part of the Far-Ultraviolet M-dwarf Evolution Survey (FUMES), the Hubble Space Telescope has observed 10 early-to-mid M dwarfs with ages ranging from $\sim$24 Myr to several Gyrs to evaluate how the incident UV radiation evolves through the lifetime of exoplanetary systems. We reconstruct the intrinsic Ly$\alpha$ profiles from STIS G140L and E140M spectra and achieve reconstructed fluxes with 1-$\sigma$ 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 Ly$\alpha$ 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 Ly$\alpha$ wings may be used to estimate the chromospheric electron density. The Ly$\alpha$ reconstructions on the G140L spectra are the first attempted on $\lambda/\Delta\lambda\sim$1000 data. We find that the reconstruction precision is not correlated with SNR of the observation, rather, it depends on the intrinsic broadness of the stellar Ly$\alpha$ 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.