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X-ray through infrared spectral energy distributions (SEDs) are essential for understanding a star's effect on exoplanet atmospheric composition and evolution. We present a catalog of panchromatic SEDs, hosted on the Barbara A. Mikulski Archive for Space Telescopes (MAST), for 11 exoplanet hosting stars which have guaranteed JWST observation time as part of the ERS or GTO programs but have no previous UV characterization. The stars in this survey range from spectral type F4-M6 (0.14-1.57 M$_\odot$), rotation periods of ~4-132 days, and ages of approximately 0.5-11.4 Gyr. The SEDs are composite spectra using data from the Chandra X-ray Observatory and XMM-Newton, the Hubble Space Telescope, BT-Settl stellar atmosphere models, and scaled spectra of proxy stars of similar spectral type and activity. From our observations, we have measured a set of UV and X-ray fluxes as indicators of stellar activity level. We compare the chromospheric and coronal activity indicators of our exoplanet-hosting stars to the broader population of field stars and find that a majority of our targets have activity levels lower than the average population of cool stars in the solar neighborhood. This suggests that using SEDs of stars selected from exoplanet surveys to compute generic exoplanet atmosphere models may underestimate the typical host star's UV flux by an order of magnitude or more, and consequently, that the observed population of exoplanetary atmospheres receive lower high-energy flux levels than the typical planet in the solar neighborhood., 28 pages, 10 figures, 7 tables, accepted for publication in AJ

Observations of H I Lyman alpha, 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, Lyman alpha observations are notoriously challenging due 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 Lyman alpha emission line away from much of the interstellar attenuation, allowing the line core to be directly observed. We detect self-reversal in the Lyman alpha 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 Lyman alpha line core, but the differences are significant enough that Mg II cannot be used directly as an intrinsic Lyman alpha template during reconstructions. We show that reconstructions that neglect self-reversal could overestimate intrinsic Lyman alpha 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 sub-solar metallicity; a larger sample size is needed to determine how sensitive these results are to these factors., Comment: 17 pages, 6 figures, accepted to ApJ

The HI Ly$��$ (1215.67 $\unicode{xC5}$) emission line dominates the far-UV spectra of M dwarf stars, but strong absorption from neutral hydrogen in the interstellar medium makes observing Ly$��$ challenging even for the closest stars. As part of the Far-Ultraviolet M-dwarf Evolution Survey (FUMES), the Hubble Space Telescope has observed 10 early-to-mid M dwarfs with ages ranging from $\sim$24 Myr to several Gyrs to evaluate how the incident UV radiation evolves through the lifetime of exoplanetary systems. We reconstruct the intrinsic Ly$��$ profiles from STIS G140L and E140M spectra and achieve reconstructed fluxes with 1-$��$ uncertainties ranging from 5% to a factor of two for the low resolution spectra (G140L) and 3-20% for the high resolution spectra (E140M). We observe broad, 500-1000 km s$^{-1}$ wings of the Ly$��$ line profile, and analyze how the line width depends on stellar properties. We find that stellar effective temperature and surface gravity are the dominant factors influencing the line width with little impact from the star's magnetic activity level, and that the surface flux density of the Ly$��$ wings may be used to estimate the chromospheric electron density. The Ly$��$ reconstructions on the G140L spectra are the first attempted on $��/����\sim$1000 data. We find that the reconstruction precision is not correlated with SNR of the observation, rather, it depends on the intrinsic broadness of the stellar Ly$��$ line. Young, low-gravity stars have the broadest lines and therefore provide more information at low spectral resolution to the fit to break degeneracies among model parameters., Accepted to AAS Journals, Paper 2 of series