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

¹ Zentrum für Astronomie und Astrophysik, TU Berlin, Sekr. PN 8-1, Hardenbergstraße 36, 10623 Berlin, Germany
² Research School of Astronomy and Astrophysics, Australian National University, Cotter Road, Weston ACT 2611, Australia e-mail: wood@mso.anu.edu.au

Corresponding author: He. Richter, richter@astro.physik.tu-berlin.de

Received: 29 July 2002
Accepted: 16 December 2002

Abstract

Our observations of cool, shock penetrated, expanding atmospheres of M-type Mira variables (Richter & Wood [CITE]) 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.