Formation of the outer rings of Supernova 1987A

The small width and the high density contrast of the observed outer rings around SN 1987A pose an outstanding problem not explainable by standard models of blue and red supergiant wind interaction. We propose a resolution of this 'mystery' by a new model for the formation of the outer rings in which the ionization and heating of the originally cool red supergiant wind induce hydrodynamic motions that naturally lead to ring formation. The model allows one to estimate an evolutionary age of order 15 000 yr after ionization for the present ring system, and indicates a modest density variation between pole and equator in the original red supergiant wind. A similar physical model may be relevant to planetary nebulae.