Rising Near-ultraviolet Spectra in Stellar Megaflares

Flares from M dwarf stars can attain energies up to 104 times larger than solar flares but are generally thought to result from similar processes of magnetic energy release and particle acceleration. Larger heating rates in the low atmosphere are needed to reproduce the shape and strength of the observed continua in stellar flares, which are often simplified to a blackbody model from the optical to the far-ultraviolet (FUV). The near-ultraviolet (NUV) has been woefully undersampled in spectral observations despite this being where the blackbody radiation should peak. We present Hubble Space Telescope NUV spectra in the impulsive phase of a flare with E TESS approximate to 7.5 x 1033 erg and a flare with E TESS approximate to 1035 erg and the largest NUV flare luminosity observed to date from an M star. The composite NUV spectra are not well represented by a single blackbody that is commonly assumed in the literature. Rather, continuum flux rises toward shorter wavelengths into the FUV, and we calculate that an optical T = 104 K blackbody underestimates the short-wavelength NUV flux by a factor of approximate to 6. We show that rising NUV continuum spectra can be reproduced by collisionally heating the lower atmosphere with beams of E greater than or similar to 10 MeV protons or E greater than or similar to 500 keV electrons and flux densities of 1013 erg cm-2 s-1. These are much larger than the canonical values describing accelerated particles in solar flares.