2025ApJ...994..140G

The role of nonthermal proton energy transportation during solar and stellar flares is largely unknown; a better understanding of this physical process will allow us to rectify longstanding deficiencies in flare models. One way to detect the presence of nonthermal protons during flares is through the Orrall-Zirker (OZ) effect, proposed by F. Q. Orrall & J. B. Zirker, whereby an enhanced red wing appears in hydrogen emission lines (e.g., Lyman-α at 1215.67 Å). We analyze archival Hubble Space Telescope/Cosmic Origins Spectrograph G130M (1060-1360 Å) observations of the young M dwarf AU Microscopii (AU Mic) to search for evidence of the OZ effect during the impulsive phase of six stellar flares with ${E}_{{\rm{flare}}}\approx 1{0}^{30-31}$ erg. While we found nondetections of the OZ effect, we note there is a pronounced blue enhancement in several C II and C III emission lines during one of the high-energy flares. We propose that either filament eruptions or chromospheric evaporation could be the mechanism driving this observed blue enhancement. We compare the far-ultraviolet (FUV) spectra to one-dimensional radiative-hydrodynamic stellar flare models, which are unable to reproduce the blue enhancement and broadening in these cool flare lines. By completing a line-by-line analysis of the FUV spectrum of AU Mic, we provide further constraints on the physical mechanisms producing stellar flares on M dwarfs.