6 Summary

The IR emission was analysed in terms of thermal emission from dust, collisionally heated by the shocked gas behind the blast wave that is expanding into the HII-region interior to the thick inner ring. We used a realistic grain model with a grain size distribution ${\rm d}n\propto a^{-k}{\rm d}a$ including stochastically heating of small grains. For the shocked gas we considered two different models corresponding to the downstream densities $n_{\rm H}=300~{\rm cm^{-3}}$(model I) and $n_{\rm H}=600~{\rm cm^{-3}}$ (model II). The conclusions for the shocked CSM are as follows:

1.

The luminosity of the dust emission is found to range from 2.91 to $3.35\times 10^{28}$ W (according to assumed grain composition and plasma density).

2.

The dust is most likely composed either of a silicate-iron or a silicate-graphite mixture or of pure graphite.

3.

Considering only cases with a maximum grain size not larger than $0.25~\mu{\rm m}$ the dust masses are, independent of the assumed grain composition and size distribution, $(0.9~{\rm to} ~1.1)\times 10^{-6}~{M_{\odot}}$ (model I) and $(0.45~{\rm to}~0.67)\times 10^{-6}$ (model II). This corresponds for a gas mass of $1.0\times 10^{-2}/n_{\rm H}[300~{\rm cm^{-3}}]~{M_{\odot}}$ to dust-to-gas ratios in the range $(0.9~{\rm to}~ 1.3)\times 10^{-4}$. For larger maximum grain sizes a larger range of the dust-to-gas ratio of up to $2.2\times 10^{-4}$ are admitted by the data.

4.

For the LMC abundances of Russell & Dopita (1992) the maximum fraction of iron condensed into grains in the shocked CSM is $32\%$ (model I) or $45\%$ (model II).

We have calculated the grain destruction due to the UV-flash of the supernova outburst and subsequent sputtering downstream of the blast wave. From this we have deduced dust properties in the CSM prior to the supernova. Our conclusions are:

1.

Pure graphite solutions for the pre-supernova CSM are underabundant in dust by an order of magnitude compared with estimates for the abundance of carbon grains in the winds of LMC-carbon stars.

2.

Solutions for silicate-iron and silicate-graphite mixtures in the pre-supernova CSM are consistent with the expected dust-to-gas ratios of $\sim$$0.3\%$ in the winds of oxygen rich LMC stars provided the maximum grain size was less than $\sim$ $0.1~\mu{\rm m}$.

Acknowledgements

The work was supported by Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR) through the projects "50 OR 9702'' and "50 OR 99140''. We have made use of the ROSAT Data Archive of the Max-Planck-Institut für extraterrestrische Physik (MPE) at Garching, Germany. We like to thank the referee Dr. S. Colgan for his helpful suggestions.