DYNAMIC DEVELOPMENT OF A RING-LIKE STRUCTURE IN THE SUPERNOVA-1987A NEBULA

CONTINUING observations of the nebula around SN1987A, which is about 3 arcseconds across, show it to be surprisingly regular. The brightest part of it is a highly axisymmetric structure, perhaps a simple ring 1-4. This structure, which may be related to similar structures in planetary nebulae and around Wolf-Rayet stars, poses a challenge for current nebular formation theories. Here we discuss the formation of an axisymmetric nebula in terms of the interaction between the slow wind ejected by the progenitor of SN1987A while it was a red supergiant (RSG) and the subsequent fast wind produced by the same star after it had evolved into a blue supergiant (BSG). In our model, an initial small asymmetry in the RSG wind, due to rotation of the red giant, is amplified by the action of the fast BSG wind. Structures that are significantly axisymmetric, and perhaps even ring-like, can be produced. We suggest that such phenomena should be common in the formation of nebulae, as evidenced by the fact that only a small fraction of the observed circumstellar nebulae appear to be spherically symmetric.