2 Observations

2.1 ISO-SWS observations

The mid-InfraRed (IR) wavelength range is rich with dust features, especially silicate features. The Short Wavelength Spectrometer (SWS) on board of ISO (de Graauw et al. 1996) has been a powerful instrument to reveal and study dust features (e.g. Waelkens et al. 1999). The SWS observation of Cas A, discussed here, was performed on December 5th 1997. The observation consists of a low-resolution full grating scan from 2.38 to 45.2$\mu$m. The position on the sky of the SWS slit is shown in Fig. 1.

Figure 1:  Optical image of the northern part of the Cas A SNR in the [OIII] lines at 4959 and 5007 Å, obtained with the SIS instrument mounted on the Canada France Hawaii Telescope (CFHT) on August 25th 1998. The white rectangle indicates the size and the position of the ISO-SWS slit aperture of the observations discussed in this paper

The data reduction was first performed using the Observer SWS Interactive Analysis (OSIA) package. Given that the Cas A flux is close to SWS sensitivity limit, additional data reduction was needed. Indeed, the data scans are corrupted with random "jumps'' in the signal, originating probably from high energy particles hitting the detectors; these "jumps'' may create long term shifts in the signal and degrade seriously the quality of the dark. To deal with the jump problem, we have used a package developed under SIA¹(Spectroscopic Interactive Analysis) by Pantin et al. (1999). Even with this careful data reduction, the signal to noise ratio is high enough only in the 16-29 $\mu$m wavelength range. In this wavelength range, the aperture of the SWS slit is equal to 14 $^{\prime\prime}$ by 27 $^{\prime\prime}$. The resulting spectrum is displayed in Fig. 2. The spectrum is similar in shape and flux level to the spectrum N3 discussed in ADM99. The slit positions on the sky corresponding to the two spectra were very close, so that, given the pointing uncertainties of ISO, the main emitting area is probably contained in both slit apertures.

Figure 2: Full line: ISOCAM (6-16 $\mu$m) and ISO-SWS (16-29$\mu$m) fluxes obtained in the aperture plotted in Fig. 1, as a function of the wavelength. The uncertainties are of $\pm$6mJy on CAM data and $\pm$1Jy on SWS data. In addition to the continuum emission that is discussed in the paper, we identify forbidden ionic line emission from [ArII] (7.0$\mu$m), [ArIII] (9.0$\mu$m), [SIV] (10.5$\mu$m), [NeII] (12.8$\mu$m), [NeV] (14.3$\mu$m, very weak), [NeIII] (15.5$\mu$m) and [OIII] (25.9$\mu$m). The dashed line is the fit of the continuum emission with Mg-protosilicates heated at 169K, as proposed by ADM99

2.2 ISOCAM observations

The spectrum at wavelengths shorter than 16$\mu$m was obtained with ISOCAM, the camera on board of ISO (Cesarsky et al. 1996). The ISOCAM observations were performed on December 5th 1996, with a pixel field of view of 6 $^{\prime\prime}$ and a corresponding total field of view of $3\hbox{$^\prime$ }\times 3\hbox{$^\prime$ }$. The field was centered on the northern part of the remnant. For each pixel, we have a spectrum from 6 to 16.5 $\mu$m, obtained by rotating the Circular Variable Filter (CVF) of ISOCAM; the spectral resolution obtained that way is $\lambda/\Delta\lambda\simeq$ 40. The data reduction was performed with the Cam Interactive Analysis (CIA) package. We have reconstructed the 7 to 16$\mu$m fluxes in the 14 $^{\prime\prime}$ by 27 $^{\prime\prime}$ SWS aperture, by co-adding the ISOCAM fluxes in this area. The resulting spectrum is shown in Fig. 2. Note that no re-scaling was applied to connect the SWS spectrum and the ISOCAM spectrum.