Predicting The Extreme Ultraviolet Radiation Environment Of The TRAPPIST-1 System
Abstract
TRAPPIST-1 is an ultra-cool M8 dwarf star that hosts seven transiting short-period terrestrial sized planets whose atmospheres will be probed by the James Webb Space Telescope. The high energy radiation that these close-in planets are exposed to strongly impacts the characteristics of their atmospheres, but these wavelengths are difficult to observe due to geocoronal contamination and interstellar attenuation. On account of these observational restrictions, a stellar atmosphere model may be used to compute the full ultraviolet stellar spectrum, including the extreme ultraviolet (EUV; 100 - 912 Å). Here we present semi-empirical non-LTE model spectra of TRAPPIST-1 that span EUV to infrared wavelengths constructed 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 Lyα 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, but EUV fluxes that span two orders of magnitude. In this, we 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 an order of magnitude higher. Based on studies using the previously predicted radiation levels to analyze the stability of the TRAPPIST-1 planet atmospheres, results from this work further suggest that these planets likely do not have much liquid water on their surfaces.
- Publication:
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American Astronomical Society Meeting Abstracts #233
- Pub Date:
- January 2019
- Bibcode:
- 2019AAS...23334108P