Extreme-ultraviolet flare activity in late-type stars

Extreme Ultraviolet Explorer Deep Survey observations of cool stars (spectr al type F to M) have been used to investigate the distribution of coronal f lare rates in energy and its relation to activity indicators and rotation p arameters. Cumulative and differential flare rate distributions were constr ucted and fitted with different methods. Power laws are found to approximat ely describe the distributions. A trend toward flatter distributions for la ter type stars is suggested in our sample. Assuming that the power laws con tinue below the detection limit, we have estimated that the superposition o f Bares with radiated energies of about 10(29)-10(31) ergs could explain th e observed radiative power loss of these coronae, while the detected Bares are contributing only approximate to 10%. Although the power-law index is n ot correlated with rotation parameters (rotation period, projected rotation al velocity, Rossby number) and only marginally with the X-ray luminosity, the flare occurrence rate is correlated with all of them. The occurrence ra te of flares with energies larger than 10(32) ergs is found to be proportio nal to the average total stellar X-ray luminosity. Thus, energetic flares o ccur more often in X-ray bright stars than in X-ray faint stars. The normal ized occurrence rate of flares with energies larger than 1032 ergs increase s with increasing L-X/L-bol and stays constant for saturated stars. A simil ar saturation is found below a critical Rossby number. The findings are dis cussed in terms of simple statistical hare models in an attempt to explain the previously observed trend for higher average coronal temperatures in mo re active stars. It is concluded that flares can contribute a significant a mount of energy to coronal heating in active stars.