A merger model for SN 1987A.

We propose that the progenitor of SN 1987A was the product of the merger of two binary components in a common envelope that formed as a result of a dynamically unstable mass-transfer phase. We consider two cases, one in which the primary (presupernova) star is on the first red-giant branch, and one in which it is on the asymptotic giant branch, when the common-envelope phase commences. While in the first case the merger would always have been completed before the supernova progenitor exploded while the stars were still in the process of spiraling into the center of the common envelope. We discuss how the latter scenario might might lead to an exotic postsupernova binary with properties that could account for the ~8 hr periodicity in the ~2 kHz pulse frequency of the supernova pulsar (Kristian et al., 1989). We also show how both scenario may explain all of the major observational features of this supernova event, including its most striking anomalies (particularly the blue color of the apparent progenitor Sk -69^deg^202).