2011A&A...536A..62L

We search for highly energetic activity phenomena in a small sample of late-type main-sequence stars in the far ultraviolet (FUV) using data from the Far Ultraviolet Spectroscopic Explorer (FUSE). Because FUSE allows a simultaneous photometric and spectroscopic analysis, we are able to analyze variations in the light curves (flares) and possible, activity-related signatures (line asymmetries, enhancements, and shifts) in the spectra. Furthermore, the computation of the well-known density-sensitive line ratio CIII(λ1176Å)/CIII(λ977Å) is also possible, and allows the investigation of its dependence on stellar activity. Three late-type main-sequence stars found in the FUSE archive (HD 36705, HD 197481, and Gl 388) show flares in their light curves. We find no obvious Doppler shifts in the brightest lines of these stars, but the OVI(λ1032Å) transition region line of AD Leo shows a blue wing enhancement one spectrum after a flare event. This emission feature is shifted by ~-84 km/s from the line core. We can exclude that the spectral feature was caused by a gas cloud co-rotating with the star and favor an interpretation of a mass ejection. In addition we find an increase of the CIII(λ1176Å)/CIII(λ977Å) line ratio during all detected flares. We compare this finding to the Sun using solar full-disk spectra from the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) and SOlar Radiation and Climate Experiment (SORCE) missions, and find that powerful flare events also show an increased CIII(λ1176Å)/CIII(λ977Å) line ratio but this result is of low statistical significance. Owing to a lack of perfectly temporally coinciding TIMED/SORCE spectra and a low temporal resolution (∼15 spectra per day) it is not possible to distinguish clearly if this increase is caused by the flares or by the related mass ejections.