A Model for the Outer Rings of Sn 1987a

I propose a model for the formation of the two outer rings of SN 1987A. The main new ingredient is a short-lived, few×10 years, intermediate wind, of velocity ∼ 100 km s and total mass of ∼ 10M⊙−few×10 M⊙, which is concentrated near the equatorial plane. This intermediate wind was formed during the few orbital periods when a binary companion entered the envelope of the progenitor of SN 1987a, when the latter was a red supergiant. The intermediate wind formed a hollowed torus with a horseshoe-shaped cross section. The inner regions at the ends of the horseshoe torus are the two outer rings of SN 1987A. The other regions of the torus were cleaned by the fast wind blown after the progenitor turns into a blue giant. The proposed scenario accounts for: (i) slow motion of the outer rings; (ii) the high density contrast of the outer rings to their surroundings; (iii) the presence of only two outer rings; (iv) the displacement of the rings relative to the central star; (v) the fact that no planetary nebulae have been observed to have a single pair of outer rings.