Multi-site continuous spectroscopy. 2: Spectrophotometry and energy budget of exceptional white-light flares on HR 1099 from the MUSICOS 89 campaign

We report results from the December 89 multi-site continuous observing campaign (MUSICOS 89) dedicated to the study of surface active structures and flares on the RS CVn-type system HR 1099 (= V711 Tau). This system has been observed by up to 17 telescopes around the globe during this campaign. We obtained complete phase coverage for Doppler imaging of photospheric spots. Quasi-simultaneously, we observed the modulation of Ca II K line profile due to chromospheric plage regions. At least two exceptional white-light flares on 14 Dec. 15:00 UT and 15 Dec. 1:00 UT (the largest such optical flare episode ever reported in a RS CVn system) were detected photometrically with typical rise and decay times of 60-90 min, and with remarkable spectral dynamic signatures in H alpha, with longer decay time scale. Equivalent colours, temperature excesses and projected flare areas (0.55 and 0.89 solar disc areas) were derived for the two optical flares. We estimate the energy budget for these two events, with respective peak intensities of radiative tosses of 1.65 and 14 10(33) erg s(-1) and integrated losses over the white-light event duration of a few hours of 8 10(36) and 10(38) ergs (in the 3100-5900 Angstrom range), indicating a total energy balance several times these values. The emission was also measured in the H alpha and H beta lines during these flares with a ratio of flare optical emission over Balmer emission 3-4 times larger compared to other flares on dwarfs. More than one day after the last white-light flare, part of the flare decay phase was also measured with IUE in UV lines of low and high excitation; the extrapolated transition region EUV losses are found similar to the derived Balmer line losses. Both flares were shown to occur near the limb. We derive their physical area, and estimate their densities, column mass, and penetration depth. From Balmer line broadening and flows, we derive a kinetic energy budget comparable to the radiative losses. We discuss a possible magnetic energy budget and the interpretation in terms of filament ejection occurring over a magnetic arcade. These observations bring a new understanding and questions about energy transport mechanisms in stellar flares.