On the shock-induced variability of emission lines in M-type Mira variables - II. Fe II and [Fe II] emission lines as a diagnostic tool

Our observations of cool, shock penetrated, expanding atmospheres of M-type Mira variables (Richter & Wood 2001) have suggested that the emission lines of Fe II and [Fe II] can serve as an excellent diagnostic tool to study the hydro- and thermodynamical conditions in the shocked regions close to the photosphere of these stars. Here we present a series of detailed NLTE (Non Local Thermodynamic Equilibrium) radiative transfer calculations, which have been performed on structures resulting from thermodynamical models of periodic shock waves, in order to calculate the emergent Fe II and [Fe II] emission line fluxes and to analyse the conditions which lead to their formation. Our basic parameter studies reveal that the ionised iron lines originate from the hot post-shock zone and that they are in fact emitted close to the star's photosphere. Furthermore, the modelling of the Fe II and [Fe II] emission line fluxes determine a specified limit of the shock velocity amplitude and pre-shock density for the innermost shocks. This offers an unique possibility to determine the thermodynamical conditions in the inner dust formation zone and thereby will shed some light on the basic mechanism of dust formation in M-type Mira stars.