Far-Ultraviolet Emission Line Investigation of Flares on AU Mic

TL;DR

This study investigates FUV emission lines during flares on AU Mic to detect non-thermal protons via the OZ effect, finding non-detections but notable blue enhancements linked to flare mechanisms, and comparing observations with models.

Contribution

It provides the first search for the OZ effect in stellar flares on AU Mic and offers insights into flare dynamics through spectral analysis and model comparisons.

Findings

No evidence of the OZ effect was detected.

Blue enhancements in C II and C III lines were observed during a high-energy flare.

Current models cannot reproduce the observed blue line features.

Abstract

The role of non-thermal 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 non-thermal protons during flares is through the Orrall-Zirker (OZ) effect, proposed by Orrall & Zirker (1976), whereby an enhanced red wing appears in hydrogen emission lines (e.g., Lyman-$\alpha$ at 1215.67 angstroms). We analyze archival Hubble Space Telescope/Cosmic Origins Spectrograph G130M (1060 - 1360 angstroms) observations of the young M dwarf, AU Mic to search for evidence of OZ effect during the impulsive phase of six stellar flares with $E_\textrm{flare} \approx 10^{30 - 31}$ erg. While we found non-detections 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 1D 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.