SN 1987A: Rotation and a Binary Companion

Abstract

In this paper we provide a possible link between the structure of the bipolar nebula surrounding SN 1987A and the properties of its progenitor star. A wind-blown bubble (WBB) scenario is employed in which a fast, tenuous wind from a blue supergiant expands into a slow, dense wind expelled during an earlier red supergiant phase. The bipolar shape develops because of a pole-to-equator density contrast in the slow wind (i.e., the slow wind forms a slow torus). We use the wind compressed disk (WCD) model of Bjorkman & Cassinelli to determine the shape of the slow torus. In the WCD scenario, the shape of the torus is determined by the rotation of the progenitor star. We then use a self-similar semianalytical method for WBB evolution to determine the shape of the resulting bipolar nebula. We find that the union of wind-compressed disk and bipolar WBB models allows us to recover the salient properties of the circumstellar nebula of SN 1987A. In particular, the size, speed, and density of the inner ring of SN 1987A are easily reproduced in our calculations. An exploration of parameter space shows that the red supergiant progenitor must have been rotating at 0.3 of its breakup speed. We conclude that the progenitor was most likely spun up by a merger with a binary companion. Using a simple model for the binary merger, we find that the companion is likely to have had a mass of 0.5 M?.