Fine-structure infrared lines from the Cassiopeia A knots
  Aims: Archival observations of infrared fine-structure lines of the young Galactic supernova remnant Cassiopeia A allow us to test existing models and determine the physical parameters of various regions of the fast-moving knots (FMKs), which are metal-dominated clouds of material ejected by the supernova explosion. Methods: The fluxes of the far-infrared [O i] and [O iii] lines are extracted from the previously unpublished archival ISO data. The archival Spitzer data are used to determine the fluxes of the O, Ne, Si, S, Ar and Fe ion fine-structure lines originating in the FMKs. The ratios of these line fluxes are used for the plasma diagnostics. We also determine the infrared line flux ratios to the optical [O iii] 5007 A line in the knots having previously measured reddening. Additionally, we analyze several optical and near-infrared observations of the FMKs to obtain clearer insight into the post-shock photoionized region (PIR) structure. Results: We show that the infrared oxygen line flux predictions of all existing theoretical models are correct only to within a factor of several. For the models to reproduce the observations it is essential to include the electron conductivity and effects of the dust. Detailed analysis of the diagnostic line flux ratios allows us to qualitatively confirm the general model of the FMK emission and to determine observationally the physical conditions in the PIR after the shock front. We infer that the pre-shock cloud densities most probably constitute several hundred particles per cm^3. We also determine the Cas A luminosities in the infrared continuum and lines. We show that accounting for the charge exchange processes in the post-shock PIR allows us to reproduce most of the relevant spectral line ratios even in the frame of a single-temperature model of this region. We also estimate its plasma parameters, thickness, and carbon abundance. 
D. Docenko and R. A. Sunyaev