The gas - dynamic model of impulsive stellar flares

We investigate the response of a plasma in a magnetically confined loop to intense impulsive energy release during stellar flares. We carry out a numerical simulation of gas-dynamic processes in an approximation to a single-fluid, two-temperature, plasma with a possible distinction between the ion and electron temperatures taken into account. We present here results of the modelling for an initial model of the red dwarf atmosphere including the photosphere, the chromosphere, the transition region and the corona. Tenuous layers of the upper chromosphere, which usually exist in quiescent regions on red dwarfs are also included in this initial model. This is a fertile field for development of our understanding of the process of explosive evaporation. Heating of the plasma is due to a hard electron beam with an energy of 3 · 10 11 erg cm −2 s −1 ; this value is based on an analysis of the soft X-ray data for stellar flares. The use of a new numerical technique reveals basic features of the gas-dynamic processes for a single, elementary, heating lasting 10 s. In the first 0.1-0.2 s, the plasma is heated strongly in the upper chromospheric layers, followed by two disturbances which subsequently propagate downward and upward from the high-pressure region formed. A flow quickly follows, with a temperature jump which moves slowly downwards, and ahead of which travels a radiative shock wave. Also, hot gas moves outwards from the region of the temperature jump. Modelling allows us to determine the physical conditions at the source of the emission in different spectral regions, and, in particular, it provides evidence for the thermal origin of optical emission from stellar flares. We discuss possibilities for the interpretation of observational data of real stellar flares which consist of a set of elementary events. This modelling has the advantage that the behaviour of the optical, EUV and soft X-ray radiation can be explained simultaneously. Our gas-dynamic modelling may be applied to impulsive stellar flares with amplitudes ∆U < 3 m , i.e. until thermal conduction fluxes are smaller than the saturated ones and the return Send offprint requests to: M.M. Katsova current doesn't limit the penetration of accelerated electrons into the chromosphere.