Dust yields in clumpy supernova shells: SN 1987A revisited

We present a study of the effects of clumping on the emergent spectral energy distribution (SED) from dusty supernova (SN) shells illuminated by a diffuse radiation source distributed throughout the medium. These models are appropriate for Type II SNe older than a few hundred days, when the energy input is dominated by gamma-rays from (56)Co decay. The fully 3D radiation transport problem is solved using a Monte Carlo code, MOCASSIN, and we present a set of models aimed at investigating the sensitivity of the SEDs to various clumping parameters. We find that, contrary to the predictions of analytical prescriptions, the combination of an optical and infrared (IR) observational data set is sufficient to constrain dust masses even in the case where optically thick clumps are present. Using both smoothly varying and clumped grain density distributions, we obtain new estimates for the mass of dust condensed by the Type II SN 1987A by fitting the optical and IR spectrophotometric data of Wooden et al. at two epochs (day 615 and day 775). When using amorphous carbon grains, our best fits to the observational data imply that about 2.0 x 10(-4) M(circle dot) of dust had condensed in the envelope of SN 1987A by day 615 and between 2.0 x 10(-4) and 4.2 x 10(-3) M(circle dot) by day 775. We find that the absence of a silicate emission or absorption feature in the observed mid-IR spectra implies that no more than 15 per cent of the dust formed around SN 1987A can have been in the form of silicate particles. Our models require larger dust masses for the case of graphite grains, namely between 4.2 x 10(-4) and 6.6 x 10(-4) M(circle dot) at day 615 and between 4.5 x 10(-4) and 7.5 x 10(-4) M(circle dot) at day 775. From our numerical models, we derive dust masses for SN 1987A that are comparable to previous analytic clumped graphite grain mass estimates, and at least two orders of magnitude below the 0.1-0.3 M(circle dot) that have been predicted to condense as dust grains in primordial core-collapse SN ejecta. This low condensation efficiency for SN 1987A is in contrast to the case of SN 2003gd, for which a dust condensation efficiency as large as 0.12 has recently been estimated.