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On the Effects of M-Dwarf UV Variability on Detecting and Interpreting Biosignatures

Presentation #0402 in the session “Stellar Activity and Life”.

Published onMar 17, 2021
On the Effects of M-Dwarf UV Variability on Detecting and Interpreting Biosignatures

In the coming decades, telescopes like JWST, LUVOIR, HabEx, and the ground-based extremely large telescopes will be capable of capturing spectra of transiting and non-transiting exoplanets. M-dwarf stars, arguably the best candidates for detecting resident terrestrial exoplanets, exhibit substantially different ultraviolet activity compared to stars like our Sun. These differences occur in multiple dimensions including spectral class, stellar age, flare activity, broadband UV flux, and the Lyman Alpha emission line. We use a suite of photochemical, climate, and stellar models along with two atmospheric archetypes, the Pre-Industrial and Archean Earths, to analyze how differences in UV radiation impact these atmospheres, the spectra they produce, and the presence or absence of biosignatures (and false positives) in those spectra. We find that some dimensions of this UV space have little impact on the chemistry of our atmospheres below 60km. However, two of the most promising biosignature gases, methane (CH4) and ozone (O3), can exhibit order-of-magnitude changes in abundance and potentially significant changes in transit and reflectance spectra when orbiting stars of varying spectral classes, ages, and UV broadband flux levels. Most notably, in our stellar age tests with the Archean Earth orbiting an early type M-dwarf, ozone’s abundance and spectral signatures rival those seen in the Pre-Industrial Earth. The ozone in our tests, which is primarily a product of carbon dioxide (CO2) photolysis, mimics the ozone that is a byproduct of oxygenic photosynthesis, and constitutes a false positive biosignature. Therefore, an understanding of a host star’s age, activity levels, and spectral class is critical for the interpretation of ozone/methane biosignature.

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