Observations of Supernova Remnants With ISO

Abstract

Pointed observations with ISO have been made, or are planned, towards 58 supernova remnants (SNRs) in the galaxy and Magellanic clouds. The main emphasis is on detailed studies of gas, dust and synchrotron emission from selected regions within 24 well studied galactic remants. First results are available for Cas A (Lagage et al. 1996(Lagage et al. , 1997Tuffs et al. 1997;Unger et al. 1997), the Cygnus Loop (deCourchelle et al 1997;Fischera et al. 1997), RCW103 (Oliva et al. 1997Wright et al. 1997), and W44 and 3C391 (Reach andRho 1996). These are providing insights into issues such as the yield and composition of condensates in supernova ejecta, the processing of grains by adiabatic and radiative shocks, and the measurement of cold gas, inaccessible to optical observations, both from cool regions in radiative shocks and from unshocked ejecta. In this progress report these points are illustrated with reference to Cas A, the prototype of an ejecta-dominated remnant of a massive core-collapse supernova, and the Cygnus Loop and RCW103 as prime examples of radiative shocks delineating the interaction of fast blast waves with diffuse atomic and dense molecular clouds, respectively. The MIR spectrum of Cas A is very rich and continuum emission is spectrally confused with fine structure, molecular and solid state emission features over much of the wavelength range of ISOPHOT-S. A faint underlying continuum can nevertheless be discerned longwards of A ~ 7 \im , with a broad bump from 8-11 fim which may plausibly be identified with silicate. The morphological similarity between ISOCAM CVF images at A 9.8 fj,m and in the [SIV] and [Ne II] lines delineating ejecta thus provides convincing evidence for the presence of condensates in the supernova ejecta (Lagage et al. 1996), the first such detection in a galactic SNR. Also seen in the ISOPHOT-S spectra are precursor molecules of the condensates, notably SiO around 8.05/xm, raising the prospect that deeper observations could measure isotope ratios in the ejecta. In common with the other SNRs observed so far, none of the unidentified emission features commonly attributed to PAHs are apparent in the spectra towards Cas A.The brightness of the diffuse MIR counterpart to the radio and X-ray shell declines dramatically with decreasing wavelength, consistent with a paucity of emission from grains with temperatures >200K (with sizes <50nm). The brightness of the shell measured by ISOPHOT in a spectral region apparently free of spectral features around 6/imis comparable with the extrapolation of 1148