1 Introduction

SN 1987A, a supernova of Type II, made it possible for the first time to make detailed observations of the interaction of a supernova ejecta with the very innermost region of its CSM. IR measurements enable us to analyse the abundance, composition, and size distribution of the circumstellar dust grains and to study grain destruction processes such as evaporation by the UV-flash of the supernova outburst and sputtering in the shocked gas behind a very strong shock. From this, new insights into the history of the progenitor star can be obtained, providing a more complete picture of the SN 1987A.

SN 1987A is the first supernova for which the progenitor was observed prior to outburst. It has been identified as the most luminous star of the system Sanduleak $-69\hbox{$^\circ$ }202$ in the Large Magellanic Cloud (LMC) (West 1987) which had been classified as a blue supergiant (BSG) of spectral Type B3 I (Rousseau 1978). Observations with the HST showed a complex axis symmetrical structure of its CSM with a thick inner ring with a diameter of $\sim$ $1\hbox{$.\!\!^{\prime\prime}$ }6$ and two outer rings with larger diameter at each side seen with a viewing angle of $\sim$ $43\hbox{$^\circ$ }$ (Burrows et al. 1995). The analysis of dust scattered light of the supernova outburst suggest that the rings are connected with gas and dust distributed in an hour glass - shaped shell (Crotts et al. 1995).

Initially, the supernova ejecta freely expanded in the thin wind of the BSG, driving a blast wave into the CSM with a velocity of $\sim$30 000 km s^-1. The reappearance of the radio emission (Staveley-Smith et al. 1992) almost coincidentally with the appearance of soft X-ray emission detected by ROSAT (Beuermann et al. 1994; Gorenstein et al. 1994) indicated that after $\sim$1200 days the blast wave had reached denser material interior to the thick inner ring, which slowed down the shock velocity to $2900\pm 480$ km s^-1 (Gaensler et al. 1997). Chevalier & Dwarkadas (1995) supposed that this denser gas is comprised of material from a RSG phase of the progenitor of SN 1987A before it evolved into the BSG that finally exploded. They refered to this region, in which the gas should be ionised through the photon flux of the progenitor star of SN 1987A, as the "HII-region''. The interaction between the fast thin wind of the BSG and the slow moving thick wind of a RSG is also thought to be responsible for the larger structure of the CSM (Blondin & Lundqvist 1993; Martin & Arnett 1995).

In a previous paper (Fischera et al. 2002, Paper I) we presented MIR measurements, made with ISOCAM (Cesarsky et al. 1996) towards SN 1987A 11 years after the outburst. These reveal the central region around the supernova position as a resolved MIR source with an extension and orientation consistent with the elliptical projection of the thick inner ring, suggesting that the MIR emission is mainly circumstellar in origin. We found this emission is most probably from dust, collisionally heated in the shocked gas downstream of the blast wave as it expanded into the material of the HII-region interior to the thick inner ring. On a purely energetical basis, all the emission could be from condensates in the metal rich core region of the expanding ejecta (Fischera 2000). However, as argued in Paper I, an emission which arises mainly from condensates does not explain the measured extension or orientation of the MIR source.

In this paper we present detailed calculations of emission from grains in the shocked CSM to analyse the implications of the ISOCAM measurements for the abundance, composition and size distribution of the circumstellar grains. In Sect. 2 we describe the dust model used to analyse the dust properties in the shocked CSM. Quantitative results are presented in Sect. 3. In Sect. 4 we discuss probably the most important destruction processes, which are evaporation of grains through the UV-flash and sputtering in the shocked gas downstream of the blast wave. In Sect. 5 this is used to derive information on grain abundance, and composition in the CSM prior to the supernova outburst. A summary is given in Sect. 6. As in Paper I we will assume a distance of 51 kpc to the supernova.