1999A&A...343..213O

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 Aurelie spectrograph at the Observatoire de Haute Provence (OHP) in 1997. The profiles analysed are those of the Balmer, Hα and Hβ, and HeI D3 (λ 5876Å) 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 HeI D3 spectra. A large flare event lasting several days was detected in both Balmer lines and in the HeI D3 line. The energy released during this flare in Hα is of the order of 10³⁷erg, 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.