The spectral and temporal behavior of exoplanet host stars is a critical input to models of the chemistry and evolution of planetary atmospheres .
Ultraviolet photons influence the atmospheric temperature profiles and production of potential biomarkers on Earth-like planets around these stars .
At present , little observational or theoretical basis exists for understanding the ultraviolet spectra of M dwarfs , despite their critical importance to predicting and interpreting the spectra of potentially habitable planets as they are obtained in the coming decades .
Using observations from the Hubble Space Telescope , we present a study of the UV radiation fields around nearby M dwarf planet hosts that covers both FUV and NUV wavelengths .
The combined FUV+NUV spectra are publically available in machine-readable format .
We find that all six exoplanet host stars in our sample ( GJ 581 , GJ 876 , GJ 436 , GJ 832 , GJ 667C , and GJ 1214 ) exhibit some level of chromospheric and transition region UV emission .
No “ UV quiet ” M dwarfs are observed .
The bright stellar Ly \alpha emission lines are reconstructed , and we find that the Ly \alpha line fluxes comprise \sim 37 – 75 % of the total 1150 – 3100 Å flux from most M dwarfs ; \gtrsim 10 ^ { 3 } times the solar value .
We develop an empirical scaling relation between Ly \alpha and Mg ii emission , to be used when interstellar H i attenuation precludes the direct observation of Ly \alpha .
The intrinsic unreddened flux ratio is F ( Ly \alpha ) / F ( Mg ii ) = 10 \pm 3 .
The F ( FUV ) / F ( NUV ) flux ratio , a driver for abiotic production of the suggested biomarkers O _ { 2 } and O _ { 3 } , is shown to be \sim 0.5 – 3 for all M dwarfs in our sample , > 10 ^ { 3 } times the solar ratio .
For the four stars with moderate signal-to-noise COS time-resolved spectra , we find UV emission line variability with amplitudes of 50 – 500 % on 10 ^ { 2 } – 10 ^ { 3 } s timescales .
This effect should be taken into account in future UV transiting planet studies , including searches for O _ { 3 } on Earth-like planets .
Finally , we observe relatively bright H _ { 2 } fluorescent emission from four of the M dwarf exoplanetary systems ( GJ 581 , GJ 876 , GJ 436 , and GJ 832 ) .
Additional modeling work is needed to differentiate between a stellar photospheric or possible exoplanetary origin for the hot ( T ( H _ { 2 } ) \approx 2000 – 4000 K ) molecular gas observed in these objects .