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,H 2 O, O 3 , CH 4 , N 2 O, and CH 3 Cl. To observe signatures of life-O 2 /O 3 in combination with reducing species like CH 4-we find that early and active M dwarfs are the best targets of the M star grid for future telescopes. The O 2 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. N 2 O, another biosignature detectable in the IR, builds up to observable concentrations in our planetary models around M dwarfs with low UV flux. CH 3 Cl could become detectable, depending on the depth of the overlapping N 2 O 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.

As part of the Mega-MUSCLES Hubble Space Telescope (HST) Treasury program, we obtained timeseries ultraviolet spectroscopy of the M2.5V star, GJ 674. During the FUV monitoring observations, the target exhibited several small flares and one large flare (E F U V = 10 30.75 ergs) that persisted over the entirety of a HST orbit and had an equivalent duration > 30, 000 sec, comparable to the highest relative amplitude event previously recorded in the FUV. The flare spectrum exhibited enhanced line emission from chromospheric, transition region, and coronal transitions and a blue FUV continuum with an unprecedented color temperature of T c 40, 000 ± 10, 000 K. In this paper, we compare the flare FUV continuum emission with parameterizations of radiative hydrodynamic model atmospheres of M star flares. We find that the observed flare continuum can be reproduced using flare models but only with the ad hoc addition of hot, dense emitting component. This observation demonstrates that flares with hot FUV continuum temperatures and significant EUV/FUV energy deposition will continue to be of importance to exoplanet atmospheric chemistry and heating even as the host M dwarfs age beyond their most active evolutionary phases.

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Observations of H i Lyman α, the brightest UV emission line of late-type stars, are critical for understanding stellar chromospheres and transition regions, modeling photochemistry in exoplanet atmospheres, and measuring the abundances of neutral hydrogen and deuterium in the interstellar medium. Yet Lyα observations are notoriously challenging owing to severe attenuation from interstellar gas, hindering our understanding of this important emission line’s basic morphology. We present high-resolution far- and near-UV spectroscopy of five G, K, and M dwarfs with radial velocities large enough to Doppler-shift the stellar Lyα emission line away from much of the interstellar attenuation, allowing the line core to be directly observed. We detect self-reversal in the Lyα emission-line core for all targets, and we show that the self-reversal depth decreases with increasing surface gravity. Mg ii self-reversed emission-line profiles provide some useful information to constrain the Lyα line core, but the differences are significant enough that Mg ii cannot be used directly as an intrinsic Lyα template during reconstructions. We show that reconstructions that neglect self-reversal could overestimate intrinsic Lyα fluxes by as much as 60%–100% for G and K dwarfs and 40%–170% for M dwarfs. The five stars of our sample have low magnetic activity and subsolar metallicity; a larger sample size is needed to determine how sensitive these results are to these factors.