EDP Sciences
Free access
Volume 400, Number 1, March II 2003
Page(s) 319 - 328
Section Formation and evolution of planetary systems
DOI http://dx.doi.org/10.1051/0004-6361:20021862

A&A 400, 319-328 (2003)
DOI: 10.1051/0004-6361:20021862

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
He. Richter1, P. R. Wood2, P. Woitke1, U. Bolick1 and E. Sedlmayr1

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

(Received 29 July 2002 / Accepted 16 December 2002)

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.

Key words: line: formation -- shock waves -- radiative transfer -- stars: late-type -- stars: variables: general

Offprint request: He. Richter, richter@astro.physik.tu-berlin.de

© ESO 2003