Effect of UV radiation on the spectral fingerprints of Earth-like planets orbiting M stars

We model the atmospheres and spectra of Earth-like planets orbiting the entire grid of M dwarfs for active and inactive stellar models with T_eff = 2300 K to T_eff = 3800 K and for six observed MUSCLES M dwarfs with UV radiation data. We set the Earth-like planets at the 1 AU equivalent distance and show spectra from the visible to IR (0.4-20 μm) to compare detectability of features in different wavelength ranges with the James Webb Space Telescope and other future ground- and spaced-based missions to characterize exo-Earths. We focus on the effect of UV activity levels on detectable atmospheric features that indicate habitability on Earth, namely, H₂O, O₃, CH₄, N₂O, and CH₃Cl. To observe signatures of life - O₂/O₃ in combination with reducing species like CH₄ - we find that early and active M dwarfs are the best targets of the M star grid for future telescopes. The O₂ spectral feature at 0.76 μm is increasingly difficult to detect in reflected light of later M dwarfs owing to low stellar flux in that wavelength region. N₂O, another biosignature detectable in the IR, builds up to observable concentrations in our planetary models around M dwarfs with low UV flux. CH₃Cl could become detectable, depending on the depth of the overlapping N₂O feature. We present a spectral database of Earth-like planets around cool stars for directly imaged planets as a framework for interpreting future light curves, direct imaging, and secondary eclipse measurements of the atmospheres of terrestrial planets in the habitable zone to design and assess future telescope capabilities.