Dust formation in the Cassiopeia A supernova

High angular resolution (6") spectro-imaging observations of Cassiopeia A, the youngest supernova remnant of our galaxy, were performed with ISOCAM, the mid-infrared camera on board of the Infrared Space Observatory (ISO). The remnant was fully mapped with the LW8 filter (10.7-12 m), which probes mainly dust thermal emission. Emission is seen both from the blast wave region and the inner region; the northeastern jet region outside the nominal radius of the remnant is also detected. Additional spectro-imaging observations of the northern part of the remnant were done in order to better characterise the dust and its link to the gas. The gaseous ionic emission lines of sulphur (SIV at 10.5 m) and of neon (NeII at 12.8 m) were observed with the ISOCAM Circular Variable Filter at a spectral resolution of 40. The gas emission is spatially well correlated with the so-called fast moving knots seen in the optical and which are known to be made of nuclear burning products from the progenitor star. The dust continuum was observed at 9.8, 11.3 and 12 m. The LW9 filter (14-16 m) was also used. A very good spatial correlation between gas and dust emission is revealed by the observations. This is a good hint that dust formation took place in the knots and that we are observing supernova grains in the evaporating interfaces between cool optical knots and the hot supernova cavity gas.