Are Coronae of Magnetically Active Stars Heated by Flares?

Abstract

Coronal flares are promising candidates as agents for plasma heating in coronae of magnetically active stars. This Letter confronts recently reconstructed coronal emission measure distributions of solar-like stars with results from a statistical, hydrodynamic approach to flare simulations. The time evolution of a statistical set of flare energy release events, distributed in total energy as a power law as observed in the Sun or in active stars, is simulated in a family of magnetically rigid loops. The resulting time-averaged emission measure distribution is double peaked, with the hotter peak due to the cooling plasma of energetic flares in the larger loops, while the cooler peak can be understood as a dispersion effect of a quiescent isothermal plasma induced by the large number of low-energy flares (microflares). The intervening minimum occurs around 10 MK and can be explained as being due to the combined effects of the flare decay of energetic flares, the power-law increase of the flare frequency toward low-energy flares, and a nearly exponential dependence between flare peak emission measure and simultaneous temperature in a given loop. The required simulation flare frequency decreases with decreasing average coronal temperature, in agreement with the observation that the average coronal temperature is correlated with magnetic activity indicators.