1998A&A...336..281F

We study different approximations usually made to simplify the calculation of chromospheric models of M dwarf stars. The approximations we discuss in details are: the assumption that the minority species are in LTE, the omission of the contribution of numerous atomic and molecular lines (line blanketing) to the opacity, and the Complete Frequency Redistribution for the Ly_α line. We consider a ``cold" chromospheric model corresponding to a low activity dM star and a ``hot" one corresponding to a very active dMe star. We find that the assumptions under study affect more strongly the cold than the hot model. In particular the approximation of Complete Frequency Redistribution in the Ly_α line, through the statistical equilibrium and ionization equilibrium equation, changes the electron density in the high chromosphere and strongly affects other line profiles.