Properties of flares and CMEs on EV Lac: Possible erupting filament

Abstract

Flares and CMEs are very powerful events in which energetic radiation and particles are ejected within a short time. These events thus can strongly affect planets that orbit these stars. This is particularly relevant for planets of M-stars, because these stars stay active for a long time during their evolution and yet potentially habitable planets orbit at short distance. Unfortunately, not much is known about the relation between flares and CMEs in M-stars as only very few CMEs have so far been observed in M-stars. In order to learn more about flares and CMEs on M-stars we monitored the active M-star EV Lac spectroscopically at high resolution. We find 27 flares with energies between 1.6E31 and 1.4E32 erg in Halpha during 127 hours of spectroscopic monitoring and 49 flares with energies between 6.3E31 and 1.1E33 erg during the 457 hours of TESS observation. Statistical analysis shows that the ratio of the continuum flux in the TESS-band to the energy emitted in Halpha is 10.408 +/- 0.026. Analysis of the spectra shows an increase in the flux of the He II 4686 Å line during the impulsive phase of some flares. In three large flares, we detect a continuum source with a temperature between 6 900 and 23 000 K. In none of the flares we find a clear CME event indicating that these must be very rare in active M-stars. However, in one relatively weak event, we found an asymmetry in the Balmer lines of ~220 km/s which we interpret as a signature of an erupting filament.