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Ultraviolet and X-ray irradiance and flares from low-mass exoplanet host stars

Published online by Cambridge University Press:  09 September 2016

Kevin France ,
R. O. Parke Loyd  and
Alex Brown
Kevin France
Affiliation:
Laboratory for Atmospheric and Space Physics, University of Colorado, 600 UCB, Boulder, CO 80309 email: kevin.france@colorado.edu
R. O. Parke Loyd
Affiliation:
Laboratory for Atmospheric and Space Physics, University of Colorado, 600 UCB, Boulder, CO 80309 email: kevin.france@colorado.edu
Alex Brown
Affiliation:
Center for Astrophysics and Space Astronomy, University of Colorado, 389 UCB, Boulder, CO 80309
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Abstract

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The spectral and temporal behavior of exoplanet host stars is a critical input to models of the chemistry and evolution of planetary atmospheres. High-energy photons (X-ray to NUV) from these stars regulate the atmospheric temperature profiles and photochemistry on orbiting planets, influencing the production of potential “biomarker” gases. We report first results from the MUSCLES Treasury Survey, a study of time-resolved UV and X-ray spectroscopy of nearby M and K dwarf exoplanet host stars. This program uses contemporaneous Hubble Space Telescope and Chandra (or XMM) observations to characterize the time variability of the energetic radiation field incident on the habitable zones planetary systems at d ≲ 20 pc. We find that all exoplanet host stars observed to date exhibit significant levels of chromospheric and transition region UV emission. M dwarf exoplanet host stars display 30–7000% UV emission line amplitude variations on timescales of minutes-to-hours. The relative flare/quiescent UV flux amplitudes on weakly active planet-hosting M dwarfs are comparable to active flare stars (e.g., AD Leo), despite their weak optical activity indices (e.g., Ca II H and K equivalent widths). We also detect similar UV flare behavior on a subset of our K dwarf exoplanet host stars. We conclude that strong flares and stochastic variability are common, even on “optically inactive” M dwarfs hosting planetary systems. These results argue that the traditional assumption of weak UV fields and low flare rates on older low-mass stars needs to be revised.

Keywords

Low-mass starsultraviolet flaresX-ray flaresexoplanet atmospheres
Type
Contributed Papers
Information
Proceedings of the International Astronomical Union , Volume 11 , Symposium S320: Solar and Stellar Flares and their Effects on Planets , August 2015 , pp. 370 - 375
Copyright
Copyright © International Astronomical Union 2016 

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