2008A&A...484..755D

Fast-moving knots (FMK) in the Galactic supernova remnant Cassiopeia A consist mainly of metals and allow us to study element production in supernovae and to investigate shock physics in great detail. We discuss and suggest observations of a previously unexplored class of spectral lines, the metal recombination lines in optical and near-infrared bands, emitted by the cold ionized and cooling plasma in fast-moving knots. By tracing ion radiative and dielectronic recombination, collisional l-redistribution and radiative cascade processes, we compute resulting oxygen, silicon and sulphur recombination line emissivities. This allows us to determine the oxygen recombination line fluxes, based on a fast-moving knot model that predicts the existence of highly-ionized ions from moderate to very low plasma temperatures. The calculations predict oxygen ion recombination line fluxes detectable with modern optical telescopes in the wavelength range from 0.5 to 3µm. Recombination line flux ratios to collisionally-excited lines will allow us to probe in detail the process of rapid cloud cooling after the passage of a shock front, to test high abundances of [4+]O, [5+]O and [6+]O ions at low temperatures and measure them, to test existing theoretical models of FMK and to build more precise ones.