3 Dust composition

Following ADM99, we have first tried to fit the spectrum of Fig. 2 with Mg-protosilicates at a temperature of 169K; the optical constants of Mg-protosilicates have been taken from Dorschner et al. (1980). The 16-29$\mu$m part of the spectrum is well fitted, but the model predicts a 10$\mu$m feature quite different from the observed one (see Fig. 2). Another material has to be found. The choice of materials is large, so that we have used, as a starting point, the materials which are predicted to form in abundance from SN material, i.e. MgSiO₃(pyroxene) and Al₂O₃ (Kozasa et al. 1991; Kozasa & Hisato 1997).

We first focused on fitting the feature at 9.5$\mu$m. The use of MgSiO₃ leads to a better fit of the feature than Mg-protosilicates; however, the predicted flux is too low in the 7.5-8.5$\mu$m range (see Fig. 3). Whatever the various silicates we use, (olivine or pyroxene with various Mg/Fe ratio, Dorschner et al. 1995, various protosilicates, Dorschner et al. 1980, Draine and Lee astronomical silicates, Draine & Lee 1984), whatever the dust temperature is, the problem persists. The use of Al₂O₃ is of no help in that region. To solve the problem, we had to introduce another dust component made of quartz (SiO₂). As can be seen in Fig. 3, a good fit of the 9.5$\mu$m feature is obtained with a contribution of both pyroxene and quartz at a relatively high temperature (345K for pyroxene and 361K for quartz); (the optical constants of the materials are obtained from Dorschner et al. 1980 and from the database on the web site of the Jena institute, http://www.astro.uni-jena.de/Group/Subgroups/ Labor/Labor/databases.html). It is not surprising to find quartz here. Indeed, quartz can be produced with a good efficiency from SN material (Kozasa 2000, private communication). In the absence of magnesium, the formation of pyroxene through the reaction $\rm Mg_{\rm gaz}+SiO_{\rm gaz}+2O_{\rm gaz}\rightarrow MgSiO_{\rm 3~solid}$ is quenched and quartz is formed instead. If the element mixing is uncomplete, as claimed in DLC99, then there is indeed a layer inside the SN, with Si, O and lack of Mg (Woosley & Weaver 1995). In case of complete mixing, given that Si is more abundant that Mg, the formation of SiO₂ is also possible.

Figure 3: ISOCAM and ISO-SWS data fitted with pyroxene (full blue and green lines), quartz (dashed blue and green lines) and aluminium oxides (dot-dashed blue and green lines). The hot component is figured in green, and the cold component in blue. The red dashed line is the possible synchrotron continuum contribution (slope from Baars et al. 1977). The red line is the result of the sum of the contributions of the various components

The same relative quantities of quartz and pyroxene, but at a cooler temperature (83K for pyroxene and 93K for quartz), lead also to a good fit of the data in the 16-29$\mu$m wavelength range. Finally, the presence of aluminium oxides (hot at 351K and cold at 113K) accounts for the 12-13$\mu$m feature, and contributes dominantly to the continuum from 13 to 16$\mu$m. Al₂O₃ exists in different forms; the best fit was obtained with a mixture of Al₂O₃ called ISAS in the paper by Koike et al. (1995); the mixture mainly consists of $\gamma$-Al₂O₃. Note that we had to decrease by a factor of 5 the relative abundance of Al₂O₃ compared to MgSiO₃, when going from the hot component to the cold component (see next section). The resulting fit is shown in Fig. 3.