Characteristics of flares on giant stars

Context. Although late-type dwarfs and giant stars are substantially different, their flares are thought to originate in similar physical processes and differ only by a scale factor in the energy levels. We study the validity of this approach.Aims. We search for characteristics of flares on active giants, which might be statistically different from those on main-sequence stars.Methods. We used nearly 4000 flares of 61 giants and 20 stars of other types that were observed with Kepler in long-cadence mode, which is the only suitable database for this comparative study. For every flare, we derived the duration and energy and gathered stellar parameters. Correlations between the flare characteristics and various stellar parameters were investigated.Results. Strong correlations are found between the flare duration and the surface gravity, luminosity, and radii of the stars. Scaled flare shapes appear to be similar on giants and dwarfs with a 30 min cadence. The logarithmic relation of flare energy and duration is steeper for stars with lower surface gravity. Observed flares are longer and more energetic on giants than on dwarfs on average.Conclusions. The generalized linear scaling for the logarithmic relation of flare energy and duration with a universal theoretical slope of asymptotic to 1/3 should slightly be modified by introducing a dependence on surface gravity.