Predicting the Extreme Ultraviolet Radiation Environment of Exoplanets Around Low-Mass Stars

Abstract

Correct estimates of the high energy radiation environment around low-mass planet host stars are important for studying the photochemistry and stability of exoplanet atmospheres. Increased levels of stellar extreme ultraviolet (EUV; 100-912 A) radiation can lead to significant atmospheric escape and global-scale water loss on close-in exoplanets, potentially leaving them uninhabitable. While interstellar contamination makes observing in these wavelengths difficult for any star other than the Sun, a stellar atmosphere model may be used to compute the stellar EUV spectrum. In this dissertation, I present semi-empirical non-LTE synthetic spectra of M stars that span EUV to infrared wavelengths. These upper-atmosphere models are constructed using the atmosphere code PHOENIX and contain prescriptions for the chromosphere and transition region. In Chapter 3, I present a case study on the M8 star, TRAPPIST-1. I construct a set of models that reproduce UV observations of TRAPPIST-1 and other M8 stars and predict EUV fluxes either consistent with or slightly higher than estimates derived from empirical scaling relationships. Results demonstrate that the EUV emission is very sensitive to the temperature structure in the transition region. Predicted levels of high energy radiation emitted by the host star indicate that the three TRAPPIST-1 habitable zone planets likely have little liquid water on their surfaces. In Chapter 4, I compute synthetic spectra of three early M dwarf planet hosts: GJ832, GJ176, and GJ436. The models closely reproduce UV observations of the stars and predict EUV fluxes consistent with estimates calculated using other techniques. We find that the temperature structures that reproduce the observations for all three early M stars, plus TRAPPIST-1, are nearly identical, suggesting that the upper atmospheres of all M-type stars may look the same. In Chapter 5, I model the evolution of EUV radiation from M stars through time. The models are guided with GALEX UV photometry and show elevated levels of EUV flux in the early stages of a stellar lifetime, when exoplanet atmospheres are forming. I predict how the average upper atmosphere structure changes over time and present age-dependent spectra that are important for determining the stability of the habitable zone around M stars.