# HAZMAT II: Ultraviolet Variability of Low-Mass Stars in the GALEX Archive

## TL;DR

This study analyzes UV variability in low-mass stars using GALEX data, revealing significant flux fluctuations, especially in FUV, which impact exoplanet atmospheric studies and biosignature detection.

## Contribution

It provides the largest characterization of UV variability in low-mass stars, highlighting the importance of FUV/NUV flux ratios for exoplanet atmosphere interpretation.

## Key findings

- Low-mass stars are more variable in FUV than NUV.
- FUV to NUV flux ratio varies widely, affecting biosignature detection.
- FUV flux increases more during flares, impacting atmospheric modeling.

## Abstract

The ultraviolet (UV) light from a host star influences a planet's atmospheric photochemistry and will affect interpretations of exoplanetary spectra from future missions like the James Webb Space Telescope. These effects will be particularly critical in the study of planetary atmospheres around M dwarfs, including Earth-sized planets in the habitable zone. Given the higher activity levels of M dwarfs compared to Sun-like stars, time resolved UV data are needed for more accurate input conditions for exoplanet atmospheric modeling. The Galaxy Evolution Explorer (\emph{GALEX}) provides multi-epoch photometric observations in two UV bands: near-ultraviolet (NUV; 1771 -- 2831 \AA) and far-ultraviolet (FUV; 1344 -- 1786 \AA). Within 30 pc of Earth, there are 357 and 303 M dwarfs in the NUV and FUV bands, respectively, with multiple\GALEX observations. Simultaneous NUV and FUV detections exist for 145 stars in both\GALEX bands. Our analyses of these data show that low-mass stars are typically more variable in the FUV than the NUV. Median variability increases with later spectral types in the NUV with no clear trend in the FUV. We find evidence that flares increase the FUV flux density far more than the NUV flux density, leading to variable FUV to NUV flux density ratios in the \GALEX\ bandpasses.The ratio of FUV to NUV flux is important for interpreting the presence of atmospheric molecules in planetary atmospheres such as oxygen and methane as a high FUV to NUV ratio may cause false-positive biosignature detections. This ratio of flux density in the\GALEX\ bands spans three orders of magnitude in our sample, from 0.008 to 4.6, and is 1 to 2 orders of magnitude higher than for G dwarfs like the Sun. These results characterize the UV behavior for the largest set of low-mass stars to date.