THE NATURE OF THE NAPOLEON HAT NEBULA OF SN 1987A

Recent observations of the Napoleon's Hat nebula of SN 1987A show that it can be described by a bow shock on the northern side of the supernova. Because of its location close to the supernova, Napoleon's Hat is doubtless affected by the wind from the precursor. It has previously been shown that the supernova is located at the top of a cocoon-shaped dark bay. By modelling the structure and dynamical formation of Napoleon's Hat and the dark bay, we conclude that the supernova progenitor star was in motion with respect to its circumstellar environment. The dark bay was produced by the shock interaction between the interstellar medium and the energetic wind of the precursor when in its main-sequence stage. The star later evolved to a red supergiant, when its mass-loss rate was higher but the stellar wind velocity much lower than in the main-sequence stage. The star eventually ran through the shocked interstellar bubble produced in the earlier main-sequence stage, and the slow RSG wind interacted directly with the interstellar medium. Another shock was driven into the interstellar medium, and can now be identified with the Napoleon's Hat nebula. The geometries of the Napoleon's Hat nebula and of the dark bay are very nicely fitted by this model. The presence of the Napoleon's Hat nebula and the dark bay is direct proof of the BSG-RSG-BSG evolutionary model for the SN 1987A progenitor.