Atmospheric Models, Ambipolar Diiusion and Streaming Particles

The upper chromosphere and lower transition region of dMe stars. Abstract. Models for the upper chromosphere and the lower transition region of dM0e-dM2e stars are presented. The modelling procedure is based on a modiication of the semi-empirical approach useful for evaluating the eeects of prescribed transport processes to the energy balance. The statistical equilibrium equations include terms due to hydrogen ambipolar diiusion in order to investigate its importance in the ionisation and energy balance. Several spectroscopic diagnostics have been used such as the hydrogen H and Ly lines, Si II resonance multiplets near 1814 A and 1264 A and the MgII h & k lines. The analysis of the H proole in the dM1e star V1005Ori (=Gl182) indicates that the temperature gradient in the lower transition region is smaller than solar. Since the gas pressure is higher than solar, these conditions imply the validity of a weakly non-uniform approximation (e. g., the Spitzer-HH arm relation for the thermal conductivity is valid in this case). Ambipolar diiusion is found ineeective for the ionisation balance and therefore for the energy transport in the lower transition region of dMe stars. This implies that, unlike the solar case, a signiicant local dissipation of mechanical heating must be present to sustain the observed radiative emission from the lower transition region.