Infrared emission towards SN 1987A, 11 years after outburst:: Properties of the circumstellar dust

Detailed models are presented for the late epoch mid infrared (MIR) emission from collisionally heated grains in the shocked circumstellar gas around SN 1987A. Thermal dust emission from a region of moderate density interior to the thick inner ring seen with the Hubble Space Telescope (HST) is found to be a natural explanation for the MIR spectral energy distribution measured by ISOCAM. The MIR-spectrum can be reproduced by a mixture of silicate-iron or silicate-graphite grains or by a composition of pure graphite grains. A composition of pure iron grains on the other hand can be excluded and a pure silicate composition does not seem to be very likely. The dust-to-gas ratio in the interaction zone is similar to0.01%, an order of magnitude lower than estimates for dust abundances in the winds of red supergiant (RSG) stars in the LMC. This low dust abundance can be accounted for by a combination of evaporation through the UV-flash from the supernova outburst and subsequent sputtering in the shocked gas. For this explanation to hold, dust in the pre-supernova circumstellar medium (CSM) would have to have been predominantly composed of grains other than graphite, with a maximum size smaller than similar to0.1 mum.