THE UPPER CHROMOSPHERE AND LOWER TRANSITION REGION OF DME STARS - ATMOSPHERIC MODELS, AMBIPOLAR DIFFUSION AND STREAMING PARTICLES

Models for the upper chromosphere and the lower transition region of dMOe-dM2e stars are presented. The modelling procedure is based on a modification of the semi-empirical approach useful for evaluating the effects of prescribed transport processes to the energy balance. The statistical equilibrium equations include terms due to hydrogen ambipolar diffusion in order to investigate its importance in the ionisation and energy balance. Several spectroscopic diagnostics have been used such as the hydrogen Ha and Ly alpha lines, Sill resonance multiplets near 1814 Angstrom and 1264 Angstrom and the Mg II h & k lines. The analysis of the Ha profile in the dM1e star V1005 Ori (=G1 182) 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, where the departure of the free electron kinetic energy distribution from the local Maxwellian is expected to be accurately taken into account by the Chapman-Enskog theory and not to be of quantitative significance for spectroscopic work. Although ambipolar diffusion plays an important role in the transport of heat flux from the corona, this is found ineffective for the ionisation balance and insufficient to balance the hydrogen radiative losses in the lower transition region of dMe stars. This implies that, unlike the solar case, a significant local dissipation of mechanical heating must be present to sustain the observed radiative emission from the lower transition region.