ORIGIN OF THE NAPOLEON HAT NEBULA AROUND SN1987A AND IMPLICATIONS FOR THE PROGENITOR

THE emission nebula around supernova 1987 A 1,2 is recombination radiation excited by the ultraviolet flash from the supernova explosion as it travels through circumstellar material. Recent observations show it to have a complex but clearly structured morphology, which has been likened to Napoleon's hat 1. We present here a simple geometrical model for the nebula, consisting of a ring and a truncated double cone. When the effects of light travel-time are included, the model reproduces the important topological structures of the nebula and makes detailed quantitative predictions for its future appearance. In particular, the hat-shaped northern rim is simply explained as the interaction of the light front with the northern cone. To explain the origin of the double cone, we argue that the progenitor of SN1987A was in a binary system: its strong wind, colliding with a weaker wind from the companion star, created an asymptotic shock surface that was spread out into the required geometry by the rotation of the binary. This suggestion is consistent with other binary models 13 for the progenitor of this unusual supernova.