ISO – SWS spectroscopy of IC 443 and the origin of the IRAS 12 and 25 μ m emission from radiative supernova remnants ⋆

ISO–SWS spectral observations of the supernova remnant IC443 are presented. Like other radiative SNRs, this object is characterized by prominent line–emitting filaments and relatively strong IRAS 12 and 25 m emission which is commonly interpreted as thermal radiation from very small grains stochastically heated by collisions with the hot plasma behind the shock front. This interpretation is challenged by the data presented in this Letter which indicate that most of the 12 and 25 m IRAS flux is accounted for by ionized line emission (mainly [NeII] and [FeII]). This result also seems to hold for other radiative SNRs. We also discuss the possible contribution of H2 lines to the IRAS 12 m flux from the southern rim of IC443 and briefly analyze the element abundances derived from the observed ionic lines.