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Stellar ultraviolet (UV) radiation drives photochemistry, andextreme-ultraviolet (EUV) radiation drives mass loss in exoplanet atmospheres.However, the UV flux is partly unobservable due to interstellar absorption,particularly in the EUV range (100--912 A). It is therefore necessary toreconstruct the unobservable spectra in order to characterize the radiationenvironment of exoplanets. In the present work, we use a radiative transfercode SSRPM to build one-dimensional semi-empirical models of two M dwarfexoplanet hosts, GJ 832 and GJ 581, and synthesize their spectra. SSRPM isequipped with an extensive atomic and molecular database and full-NLTEcapabilities. We use observations in the visible, ultraviolet, and X-ray rangesto constrain atmospheric structures of the modeled stars. The synthesizedintegrated EUV fluxes are found to be in good agreement with otherreconstruction techniques, but the spectral energy distributions (SEDs)disagree significantly across the EUV range. More than 2/3 of the EUV flux isformed above $10^5$ K. We find that the far ultraviolet (FUV) continuumcontributes 42--54 % of the entire FUV flux between 1450--1700 A. Thecomparison of stellar structures of GJ 832 and GJ 581 suggests that GJ 832 is amore magnetically active star, which is corroborated by other activityindicators.

Semi-Empirical Modeling of the Atmospheres of the M Dwarf Exoplanet Hosts GJ 832 and GJ 581