Testing Lyα Emission-line Reconstruction Routines at Multiple Velocities in One System

<jats:title>Abstract</jats:title>
<jats:p>The 1215.67 Å H <jats:sc>i</jats:sc> Ly<jats:italic>α</jats:italic> emission line dominates the ultraviolet flux of low-mass stars, including the majority of known exoplanet hosts. Unfortunately, strong attenuation by the interstellar medium (ISM) obscures the line core in most stars, requiring the intrinsic Ly<jats:italic>α</jats:italic> flux to be reconstructed based on fits to the line wings. We present a test of the widely used Ly<jats:italic>α</jats:italic> emission-line reconstruction code <jats:sc>lyapy</jats:sc> using phase-resolved, medium-resolution STIS G140M observations of the close white dwarf–M dwarf binary EG UMa. The Doppler shifts induced by the binary orbital motion move the Ly<jats:italic>α</jats:italic> emission line in and out of the region of strong ISM attenuation. Reconstructions of each spectrum should produce the same Ly<jats:italic>α</jats:italic> profile regardless of phase, under the well-justified assumption that there is no intrinsic line variability between observations. Instead, we find that the reconstructions underestimate the Ly<jats:italic>α</jats:italic> flux by almost a factor of 2 for the lowest velocity, most attenuated spectrum, due to a degeneracy between the intrinsic Ly<jats:italic>α</jats:italic> and ISM profiles. Our results imply that many stellar Ly<jats:italic>α</jats:italic> fluxes derived from G140M spectra reported in the literature may be underestimated, with potential consequences for, for example, estimates of extreme-ultraviolet stellar spectra and ultraviolet inputs into simulations of exoplanet atmospheres.</jats:p>