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

The continuum emission of stellar flares in UV and visible bands can be enhanced by two or even three orders of magnitude relative to the quiescent level and is usually characterised by a blue colour. Thermal atmospheric models are difficult to reproduce all these spectral features. If the flaring process involves the acceleration of energetic electrons which then precipitate downwards to heat the lower atmosphere, collisional excitation and ionisation of ambient hydrogen atoms by these nonthermal electrons could be important in powering the continuum emission. To explore such a possibility, we compute the continuum spectra from an atmospheric model for a dMe star, AD Leo, at its quiescent state, when considering the nonthermal effects by precipitating electron beams. The results show that if the electron beam has an energy flux large enough (for example, F1 ∼ 10 12 erg cm s), 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 F1 ∼ 10 11 erg cm s, 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.