The role of nonthermal electrons in the optical continuum of stellar flares

We explore the possibility that the continuum emission in stellar flares is powered by nonthermal electrons accelerated during the flares. We compute the continuum spectra from an atmospheric model for a dMe star, AD Leo, at its quiescent state, when considering the nonthermal excitation and ionisation effects by precipitating electron beams. The results show that if the electron beam has an energy flux large enough, the U band brightening and, in particular, the U-B colour are roughly comparable with observed values for a typical large flare. Moreover, for electron beams with a moderate energy flux, a decrease of the emission at the Paschen continuum appears. This can explain at least partly the continuum dimming observed in some stellar flares. Adopting an atmospheric model for the flaring state can further raise the continuum flux but it yields a spectral colour incomparable with observations. This implies that the nonthermal effects may play the chief role in powering the continuum emission in some stellar flares.Comment: 6 pages, 4 figures, LaTeX (psfigs.sty), to appear in MNRA