Infrared spectroscopy of young supernova remnants heavily interacting with the interstellar medium ⋆ I . Ionized species in RCW 103

ISO spectral observations of the supernova remnant RCW103 are presented. This object is the prototype of relatively young remnants ( 103 yr) with fast shocks (vs 1000 km/s) interacting with dense interstellar medium. The spectrum is dominated by prominent lines of [NeII], [SiII], [FeII] and other low excitation species which provide, for the first time, a simple and reliable estimate of the gas abundances of refractory (Si, Fe, Ni) and non–refractory (Ne, P, S, Cl, Ar) species. Apart from nickel, all the derived abundances are close to solar, confirming that the shock has destroyed all dust grains. Like the optical nickel lines, [NiII] 6.64 m yields Ni abundances a factor '10 solar which we propose results from a large underestimation of the computed Ni+ collision strengths. The observed intensities and velocity widths of ionic lines are compatible with emission from the post–shock region alone with only a very small (if any) contribution from the photoionized precursor. This result does not agree with shock models which predict that the precursor should emit powerful line emission, especially from highly ionized species. The possible consequence of this on the modelling of Seyfert spectra is briefly discussed.