Predicting the Extreme Ultraviolet Radiation Environment of Exoplanets Around Low-Mass Stars: the TRAPPIST-1 System. (arXiv:1812.06159v1 [astro-ph.SR])

Predicting the Extreme Ultraviolet Radiation Environment of Exoplanets Around Low-Mass Stars: the TRAPPIST-1 System. (arXiv:1812.06159v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Peacock_S/0/1/0/all/0/1">Sarah Peacock</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Barman_T/0/1/0/all/0/1">Travis Barman</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shkolnik_E/0/1/0/all/0/1">Evgenya L. Shkolnik</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hauschildt_P/0/1/0/all/0/1">Peter H. Hauschildt</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Baron_E/0/1/0/all/0/1">E. Baron</a>

The high energy radiation environment around M dwarf stars strongly impacts
the characteristics of close-in exoplanet atmospheres, but these wavelengths
are difficult to observe due to geocoronal and interstellar contamination. On
account of these observational restrictions, a stellar atmosphere model may be
used to compute the stellar extreme ultraviolet (EUV; 100 – 912 AA) spectrum.
We present a case study of the ultra-cool M8 dwarf star, TRAPPIST-1, which
hosts seven transiting short-period terrestrial sized planets whose atmospheres
will be probed by the James Webb Space Telescope. We construct semi-empirical
non-LTE model spectra of TRAPPIST-1 that span EUV to infrared wavelengths (100
AA – 2.5 $mu$m) using the atmosphere code PHOENIX. These upper-atmosphere
models contain prescriptions for the chromosphere and transition region and
include newly added partial frequency redistribution capabilities. In the
absence of broadband UV spectral observations, we constrain our models using
HST Ly$alpha$ observations from TRAPPIST-1 and GALEX FUV and NUV photometric
detections from a set of old M8 stars ($>$1 Gyr). We find that calibrating the
models using both data sets separately yield similar FUV and NUV fluxes, and
EUV fluxes that range from (1.32 – 17.4) $times$ 10$^{-14}$ ergs s$^{-1}$
cm$^{-2}$. The results from these models demonstrate that the EUV emission is
very sensitive to the temperature structure in the transition region. Our lower
activity models predict EUV fluxes similar to previously published estimates
derived from semi-empirical scaling relationships, while the highest activity
model predicts EUV fluxes a factor of ten higher. Results from this study
support the idea that the TRAPPIST-1 habitable zone planets likely do not have
much liquid water on their surfaces due to the elevated levels of high energy
radiation emitted by the host star.

The high energy radiation environment around M dwarf stars strongly impacts
the characteristics of close-in exoplanet atmospheres, but these wavelengths
are difficult to observe due to geocoronal and interstellar contamination. On
account of these observational restrictions, a stellar atmosphere model may be
used to compute the stellar extreme ultraviolet (EUV; 100 – 912 AA) spectrum.
We present a case study of the ultra-cool M8 dwarf star, TRAPPIST-1, which
hosts seven transiting short-period terrestrial sized planets whose atmospheres
will be probed by the James Webb Space Telescope. We construct semi-empirical
non-LTE model spectra of TRAPPIST-1 that span EUV to infrared wavelengths (100
AA – 2.5 $mu$m) using the atmosphere code PHOENIX. These upper-atmosphere
models contain prescriptions for the chromosphere and transition region and
include newly added partial frequency redistribution capabilities. In the
absence of broadband UV spectral observations, we constrain our models using
HST Ly$alpha$ observations from TRAPPIST-1 and GALEX FUV and NUV photometric
detections from a set of old M8 stars ($>$1 Gyr). We find that calibrating the
models using both data sets separately yield similar FUV and NUV fluxes, and
EUV fluxes that range from (1.32 – 17.4) $times$ 10$^{-14}$ ergs s$^{-1}$
cm$^{-2}$. The results from these models demonstrate that the EUV emission is
very sensitive to the temperature structure in the transition region. Our lower
activity models predict EUV fluxes similar to previously published estimates
derived from semi-empirical scaling relationships, while the highest activity
model predicts EUV fluxes a factor of ten higher. Results from this study
support the idea that the TRAPPIST-1 habitable zone planets likely do not have
much liquid water on their surfaces due to the elevated levels of high energy
radiation emitted by the host star.

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