Modelling supernova line profile asymmetries to determine ejecta dust
  masses: SN 1987A from days 714 to 3604
  The late time optical and near-IR line profiles of many core-collapse
supernovae exhibit a red-blue asymmetry as a result of greater extinction by
internal dust of radiation emitted from the receding parts of the supernova
ejecta. We present here a new code, DAMOCLES, that models the effects of dust
on the line profiles of core-collapse supernovae in order to determine the
masses of newly formed dust. As noted by Lucy et al. (1989), the presence of an
extended red scattering wing in late-time line profiles can also indicate dust
formation. We find that dust-affected line profiles need not necessarily be
flux-biased towards to the blue, although the profile peak will always be
blue-shifted. We have collated optical spectra of SN 1987A from a variety of
archival sources and have modelled the evolution of the H$\alpha$ line from
days 714 to 3604, as well as that of the [OI] 6300,6363A doublet between days
714 and 1478. A variety of evidence points to the presence of clumping and we
find that our clumped dust models require significantly higher dust masses than
smoothly distributed dust models. Our line profile fits imply day 714 dust
masses of <3 $\times$ 10$^{-3}$ M$_{\odot}$ for all grain types apart from very
high albedo pure magnesium silicates, for which up to 0.07M$_{\odot}$ can be
accommodated. Large grain radii (>0.6$\mu$m) are generally required to fit the
line profiles even at the earlier epochs. We find that a large dust mass
(>0.1M$_{\odot}$) had formed by day 3604 and infer that the majority of the
present dust mass must have formed after this epoch. Our findings agree with
recent estimates from SED fits for the dust mass evolution of SN 1987A and
support the inference that the majority of SN 1987A's dust formed many years
after the initial explosion.
