Circumstellar emission and flares on FK Comae Berenices: Analysis and modelling of Balmer and He I D3 line variations

We present results on spectroscopic observations of the fast-rotating active giant FK Comae, obtained mainly with the ESA-MUSICOS spectrograph at the Isaac Newton Telescope (INT) in 1996 and 1997 and also with the Aurélie spectrograph at the Observatoire de Haute Provence (OHP) in 1997. The profiles analysed are those of the Balmer, Hα and Hβ, and He I D3 (λ 5876 Å) lines. We analyse the Balmer line variability and phase behaviour. We confirm these lines as highly variable, with excess emission that originates from extended structures and exhibits clear signs of rotational modulation. We have described the line profiles for two distinct states of activity, using different modelling approaches. Similar techniques were applied to the He I D3 spectra. A large flare event lasting several days was detected in both Balmer lines and in the He I D3 line. The energy released during this flare in Ha is of the order of 10³⁷ erg, making it the largest Hα flare reported on a cool star. Our results confirm the extreme complexity of the circumstellar environment of FK Comae. The activity level of this star is quite variable demanding different approaches to the line profile analysis.