Optical and near-infrared recombination lines of oxygen ions from Cassiopeia A knots^*

¹ Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany e-mail: dima@mpa-garching.mpg.de
² Institute of Astronomy, University of Latvia, Raiņa bulvāris 19, Riga LV-1586, Latvia
³ Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia

Received: 14 February 2008
Accepted: 21 March 2008

Abstract

Context. 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.

Aims. 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.

Methods. 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.

Results. The calculations predict oxygen ion recombination line fluxes detectable with modern optical telescopes in the wavelength range from 0.5 to 3 μm. 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 O⁴⁺, O⁵⁺ and O⁶⁺ ions at low temperatures and measure them, to test existing theoretical models of FMK and to build more precise ones.