  The Lyman-alpha emission line dominates the far-ultraviolet spectra of
late-type stars and is a major source for photodissociation of important
molecules including H2O, CH4, and CO2 in exoplanet atmospheres. The incident
flux in this line illuminating an exoplanet's atmosphere cannot be measured
directly as neutral hydrogen in the interstellar medium (ISM) attenuates most
of the flux reaching the Earth. Reconstruction of the intrinsic Lyman-alpha
line has been accomplished for a limited number of nearby stars, but is not
feasible for distant or faint host stars. We identify correlations connecting
the intrinsic Lyman-alpha flux with the flux in other emission lines formed in
the stellar chromosphere, and find that these correlations depend only
gradually on the flux in the other lines. These correlations, which are based
on HST spectra, reconstructed Lyman-alpha line fluxes, and irradiance spectra
of the quiet and active Sun, are required for photochemical models of exoplanet
atmospheres when intrinsic Lyman-alpha fluxes are not available. We find a
tight correlation of the intrinsic Lyman-alpha flux with stellar X-ray flux for
F5 V to K5 V stars, but much larger dispersion for M stars. We also show that
knowledge of the stellar effective temperature and rotation rate can provide
reasonably accurate estimates of the Lyman-alpha flux for G and K stars, and
less accurate estimates for cooler stars.
