  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 ~0.01%, 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 ~0.1 microns.
