The ring around SN 1987A and rotation of the progenitor

The ring discovered around SN 1987A is likely to be related to the rotation of the progenitor. We have studied two rotational effects of the progenitor of SN 1987A: (1) mass-shedding from the equator and (2) an aspherical mass distribution in a stellar wind due to the direction-dependent mass loss. With approximate models for a rotating star and a wind, we have obtained the following results. First, if the rotational velocity of the progenitor at the main sequence exceeds 270 km s^-1, mass-shedding can occur (i.e., the angular velocity can exceed the critical angular velocity) at the blue supergiant stage. This is due to the decreasing moment of inertia along the evolution with increasing mass concentration toward the center. However, the observed elemental abundances and the expansion velocity of the ring provide rather severe constraints on the time of mass-shedding, which suggests that the mass-shedding scenario is probably less likely the case. Second, the stellar wind combined with the rotation forms an aspherical mass distribution in the distant region from the star. The nonsphericity can be 10%-20% depending on the ratio of the rotational energy to the gravitational energy. This mild asphericity in the slow wind can be a seed of the ring formation due to the interaction with the fast wind at later stages, which should be studied with full hydrodynamical calculations of the wind-wind interaction.