THE MUSCLES TREASURY SURVEY. II. INTRINSIC LYα AND EXTREME ULTRAVIOLET SPECTRA OF K AND M DWARFS WITH EXOPLANETS

The ultraviolet (UV) spectral energy distributions (SEDs) of low-mass (K- and M-type) stars play a critical role in the heating and chemistry of exoplanet atmospheres, but are not observationally well-constrained. Direct observations of the intrinsic flux of the Ly alpha line (the dominant source of UV photons from low-mass stars) are challenging, as interstellar HI absorbs the entire line core for even the closest stars. To address the existing gap in empirical constraints on the UV flux of K and M dwarfs, the MUSCLES Hubble Space Telescope Treasury Survey has obtained UV observations of 11 nearby M and K dwarfs hosting exoplanets. This paper presents the Ly alpha and extreme-UV spectral reconstructions for the MUSCLES targets. Most targets are optically inactive, but all exhibit significant UV activity. We use a Markov Chain Monte Carlo technique to correct the observed Ly alpha profiles for interstellar absorption, and we employ empirical relations to compute the extreme-UV SED from the intrinsic Ly alpha flux in similar to 100 angstrom bins from 100-1170 angstrom. The reconstructed Ly alpha profiles have 300 km s(-1) broad cores, while >1% of the total intrinsic Ly alpha flux is measured in extended wings between 300 and 1200 km s(-1). The Ly alpha surface flux positively correlates with the Mg II surface flux and negatively correlates with the stellar rotation period. Stars with larger Ly alpha surface flux also tend to have larger surface flux in ions formed at higher temperatures, but these correlations remain statistically insignificant in our sample of 11 stars. We also present H I column density measurements for 10 new sightlines through the local interstellar medium.