Optically Quiet, But FUV Loud: Results from comparing the far-ultraviolet predictions of flare models with TESS and HST

TL;DR

This study compares FUV flare predictions from models with actual TESS and HST observations, revealing significant underestimations in many models and providing correction factors to improve FUV flare activity estimates for low-mass stars.

Contribution

It evaluates the accuracy of existing flare models against combined optical and FUV data, offering improved correction factors for FUV flare predictions in low-mass stars.

Findings

Optical+NUV blackbody models best match FUV observations.

Many models underestimate FUV emission by up to 10,000 times.

Optically quiet stars still exhibit regular FUV flares.

Abstract

The far-ultraviolet (FUV) flare activity of low-mass stars has become a focus in our understanding of the exoplanet atmospheres and how they evolve. However, direct detection of FUV flares and measurements of their energies and rates are limited by the need for space-based observations. The difficulty of obtaining such observations may push some works to use widely available optical data to calibrate multi-wavelength spectral models that describe UV and optical flare emission. These models either use single temperature blackbody curves to describe this emission, or combine a blackbody curve with archival spectra. These calibrated models would then be used to predict the FUV flare rates of low-mass stars of interest. To aid these works, we used TESS optical photometry and archival HST FUV spectroscopy to test the FUV predictions of literature flare models. We tested models for partially (M0-M2) and fully convective (M4-M5) stars, 40 Myr and field age stars, and optically quiet stars. We calculated FUV energy correction factors that can be used to bring the FUV predictions of tested models in line with observations. A flare model combining optical and NUV blackbody emission with FUV emission based on HST observations provided the best estimate of FUV flare activity, where others underestimated the emission at all ages, masses and activity levels, by up to a factor of 104 for combined FUV continuum and line emission and greater for individual emission lines. We also confirmed previous findings that showed optically quiet low-mass stars exhibit regular FUV flares.