Inferring Intrinsic Stellar EUV and Lyman-Alpha Fluxes and Their Effects on Exoplanet Atmospheres

EUV radiation is primarily responsible for driving hydrodynamic mass loss and thus determining whether an exoplanet can retain its atmosphere and water. The Lyman- $\alpha$ $\alpha $ flux is primarily responsible for photo-dissociating water and methane, and therefore plays a major role in determining the chemistry in the upper atmospheres of exoplanets. Since interstellar hydrogen absorbs much of the EUV and Lyman- $\alpha$ $\alpha $ radiation, reconstruction and theoretical techniques are needed to determine the intrinsic flux levels received by exoplanets.

We describe the techniques and their limitations for estimating the extreme ultraviolet (EUV) spectral energy distribution (10–91.2 nm) and hydrogen Lyman- $\alpha$ $\alpha $ flux (121.6 nm) emitted by host stars and incident on exoplanet atmospheres.

We evaluate how each reconstruction technique can match the observed solar EUV spectral energy distribution and Lyman- $\alpha$ $\alpha $ flux.

Each technique has its limitations, but the techniques that can reconstruct the observed solar emission and are based on stellar activity observables that are not affected by interstellar absorption should best explain the intrinsic EUV and Lyman- $\alpha$ $\alpha $ stellar emission.