The environment of the SN1987A is quite complex but also very regularly structured. Detailed analyses of direct images taken under good seeing conditions (0.3—0.8 arcsec) from the European Southern Observatory (ESO)'s New Technology Telescope (NTT) show that there are two nebular loops within the 3 arcsec environment of the SN. The inner loop is elliptical in shape. The kinematics of this loop as revealed by spectroscopic data with a spectral resolving power λ/Δλ « 30 000 provide further clues for the three dimensional structure of these two loops. The data show that the overall structure of the nebulosity can be understood by an hourglass-shaped shell with significant mass enhancement on its equatorial plane. A diffuse nebulosity called Napoleon's Hat is observed at a distance of about 5 arcsec to the north of the SN. It showed little size evolution since the first observation on Aug. 1989, until it disappeared on Jan, 1992. The Napoleon's Hat nebula appears to be a bow-shock coming from an interaction between the supernova progenitor's stellar wind and the interstellar medium, as the supernova progenitor moved through the interstellar medium with a velocity of around 5 kms-1. On an even larger scale, there is a huge dark bay of size around 100 arcsec in diameter, we suggested that this bay was also formed by interactions between the supernova progenitor and the interstellar medium.
The narrow emission lines indicate that the electron density and temperature of the emission gas in the loops are steadily decreasing. However, the temperatures inferred from the [OIII] nebular lines remained persistently higher than 2 × 104 K. They were around 104 cm −3 and 2 × 104 K on Aug. 30, 1990. In addition, a global difference in the electron density of the two major axis of the inner loop is observed in the images and spectra taken five years after the supernova explosion. We have also studied the two neighboring stars surrounding the supernova. We find that star 2 is a B star with Hα and Hβ emission lines.
The supernova circumstellar nebulosity is remarkably similar to certain types of planetary nebula, e.g. NGC2392. Asymmetric wind interaction models for the formation of planetary nebula are applied to produce a model in the form of an hourglass shaped shell. This model invokes both a slow wind while the star was in its red supergiant phase and a fast wind during the later blue supergiant stage before explosion. Although such wind interaction models fit the observed nebular structures satisfactorily, they are so idealized that they tell us little about the origin of the seed asymmetry or the amount of seed asymmetry needed to produce the observed real nebula.
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