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Title: Predicting the Extreme Ultraviolet Radiation Environment of Exoplanets around Low-mass Stars: The TRAPPIST-1 System

Abstract

The high energy radiation environment around M dwarf stars strongly affects the characteristics of close-in exoplanet atmospheres, but these wavelengths are difficult to observe because of geocoronal and interstellar contamination. Due to these observational restrictions, a stellar atmosphere model may be used to compute the stellar extreme ultraviolet (EUV; 100–912 Å) spectrum. We construct semiempirical nonlocal thermodynamic equilibrium model spectra of the ultracool M8 star TRAPPIST-1 that span EUV to infrared wavelengths (100 Å-2.5 μm) 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 Hubble Space Telescope Lyman α observations from TRAPPIST-1 and Galaxy Evolution Explorer UV photometric detections from a set of old M8 stars (>1 Gyr). We find that calibrating the models using both data sets separately yield similar far-ultraviolet and NUV fluxes, and EUV fluxes that range from (1.32–17.4) × 10 -14 ergs s -1 cm -2. The findings 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 previouslymore » published estimates derived from semiempirical scaling relationships, while the highest activity model predicts EUV fluxes a factor of 10 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.« less

Authors:
ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [2];  [3]; ORCiD logo [4]
  1. Univ. of Arizona, Tucson, AZ (United States)
  2. Arizona State Univ., Tempe, AZ (United States)
  3. Univ. of Hamburg (Germany)
  4. Univ. of Hamburg (Germany); Univ. of Oklahoma, Norman, OK (United States)
Publication Date:
Research Org.:
Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States). National Energy Research Scientific Computing Center (NERSC); Univ. of California, San Diego, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
1544081
Grant/Contract Number:  
AC03-76SF00098
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
The Astrophysical Journal (Online)
Additional Journal Information:
Journal Volume: 871; Journal Issue: 2; Journal ID: ISSN 1538-4357
Publisher:
Institute of Physics (IOP)
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; stars: activity; stars: chromospheres; stars: low-mass; ultraviolet: stars

Citation Formats

Peacock, Sarah, Barman, Travis, Shkolnik, Evgenya L., Hauschildt, Peter H., and Baron, E. Predicting the Extreme Ultraviolet Radiation Environment of Exoplanets around Low-mass Stars: The TRAPPIST-1 System. United States: N. p., 2019. Web. doi:10.3847/1538-4357/aaf891.
Peacock, Sarah, Barman, Travis, Shkolnik, Evgenya L., Hauschildt, Peter H., & Baron, E. Predicting the Extreme Ultraviolet Radiation Environment of Exoplanets around Low-mass Stars: The TRAPPIST-1 System. United States. doi:10.3847/1538-4357/aaf891.
Peacock, Sarah, Barman, Travis, Shkolnik, Evgenya L., Hauschildt, Peter H., and Baron, E. Mon . "Predicting the Extreme Ultraviolet Radiation Environment of Exoplanets around Low-mass Stars: The TRAPPIST-1 System". United States. doi:10.3847/1538-4357/aaf891. https://www.osti.gov/servlets/purl/1544081.
@article{osti_1544081,
title = {Predicting the Extreme Ultraviolet Radiation Environment of Exoplanets around Low-mass Stars: The TRAPPIST-1 System},
author = {Peacock, Sarah and Barman, Travis and Shkolnik, Evgenya L. and Hauschildt, Peter H. and Baron, E.},
abstractNote = {The high energy radiation environment around M dwarf stars strongly affects the characteristics of close-in exoplanet atmospheres, but these wavelengths are difficult to observe because of geocoronal and interstellar contamination. Due to these observational restrictions, a stellar atmosphere model may be used to compute the stellar extreme ultraviolet (EUV; 100–912 Å) spectrum. We construct semiempirical nonlocal thermodynamic equilibrium model spectra of the ultracool M8 star TRAPPIST-1 that span EUV to infrared wavelengths (100 Å-2.5 μm) 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 Hubble Space Telescope Lyman α observations from TRAPPIST-1 and Galaxy Evolution Explorer UV photometric detections from a set of old M8 stars (>1 Gyr). We find that calibrating the models using both data sets separately yield similar far-ultraviolet and NUV fluxes, and EUV fluxes that range from (1.32–17.4) × 10-14 ergs s-1 cm-2. The findings 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 semiempirical scaling relationships, while the highest activity model predicts EUV fluxes a factor of 10 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.},
doi = {10.3847/1538-4357/aaf891},
journal = {The Astrophysical Journal (Online)},
issn = {1538-4357},
number = 2,
volume = 871,
place = {United States},
year = {2019},
month = {2}
}

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