Introduction

If a supernova progenitor has undergone significant mass-loss then the expanding supernova ejecta will eventually collide with this circumstellar material (CSM). Shock waves arising from the collision will compress and heat both the ejecta and the CSM. The emission from the shocked material depends strongly on the density distributions of the ejecta and the CSM, thereby providing important information about the nature of the CSM.

SN 1987A

Images from the European Southern Observatory (ESO) (Wampler et al. 1990) and the Hubble Space Telescope (HST) (Jakobsen et al. 1991) revealed the presence of a ring-like structure at ∼ 6 × 1017 cm from SN 1987A. The outermost part of the supernova, ejecta is expanding at ∼ 104 km s−1 (Shigeyama & Nomoto 1990) and so is expected to collide with the ring ∼ 10 years after the explosion.

Hydrodynamical model

The progenitor of SN 1987A went through a red supergiant (RSG) phase, and then contracted to a blue supergiant (BSG) before the explosion (for reviews, see Arnett et al. 1989, Hillebrandt & Höflich 1989, Podsiadlowski 1992, and Nomoto et al. 1993a). This evolutionary scenario implies that the SN 1987A environment was formed as follows: the progenitor blew a stellar wind with a velocity ∼ 10 km s−1 and a mass loss rate ∼ 10−5M⊙ yr−1 during the RSG stage, and with corresponding values of ∼ 550 km s−1 and ∼ 10−6M⊙ yr−1 during the BSG stage (Lundqvist & Fransson 1991).