Extreme‐Ultraviolet Flare Activity in Late‐Type Stars

\textit{Extreme Ultraviolet Explorer} Deep Survey observations of cool stars(spectral type F to M) have been used to investigate the distribution ofcoronal flare rates in energy and its relation to activity indicators androtation parameters. Cumulative and differential flare rate distributions wereconstructed and fitted with different methods. Power laws are found toapproximately describe the distributions. A trend toward flatter distributionsfor later-type stars is suggested in our sample. Assuming that the power lawscontinue below the detection limit, we have estimated that the superposition offlares with radiated energies of about $10^{29}-10^{31}$ergs could explain theobserved radiative power loss of these coronae, while the detected flares arecontributing only $\approx 10$%. While the power-law index is not correlatedwith rotation parameters (rotation period, projected rotational velocity,Rossby number) and only marginally with the X-ray luminosity, the flareoccurrence rate is correlated with all of them. The occurrence rate of flareswith energies larger than $10^{32}$ergs is found to be proportional to theaverage total stellar X-ray luminosity. Thus, energetic flares occur more oftenin X-ray bright stars than in X-ray faint stars. The normalized occurrence rateof flares with energies larger than $10^{32}$ergs increases with increasing\lxlbol and stays constant for saturated stars. A similar saturation is foundbelow a critical Rossby number. The findings are discussed in terms of simplestatistical flare models in an attempt to explain the previously observed trendfor higher average coronal temperatures in more active stars. It is concludedthat flares can contribute a significant amount of energy to coronal heating inactive stars.Comment: 3 pages, 9 figures, accepted for publication in The Astrophysical Journal. Use of emulateapj5.sty and apjfonts.sty, LaTeX2