Formation of the circumstellar shell around SN 1987A

The nebulosity within a few arcseconds from SN 1987A has been modeled in terms of the supernova progenitor's fast wind interacting with a slow, asymmetric, previously emitted red supergiant wind. Previous models have relied on the assumption that the shocked blue supergiant wind is isobaric. We show that this approximation is unsatisfactory, and present two-dimensional time-dependent hydrodynamic calculations of the colliding winds model. From adiabatic models it is found that a ratio of equatorial to polar mass-loss rate during the red supergiant stage of at least ~20 is needed to explain the observed structure. It is shown that not only this ratio, but also the form of polar dependence on asymmetry, is important to the formation of a compact ring structure. Models are also presented in which radiative cooling of the shocked red supergiant wind is included and, at early times, also radiative cooling of the shocked blue supergiant wind. Cooling acts to lower the expansion velocity and to increase filamentation. We arrive at a viable colliding winds model for the formation of the circumstellar structure around SN 1987A consisting of a red supergiant wind with a velocity of 5 km s^-1^ and a mass-loss rate of 2 x 10^-5^ M_sun_ yr^-1^, and a blue supergiant wind with a velocity of 300 km s^-1^ and mass-loss rate of 3 x 10^-7^ M_sun_ yr^-1^. This model can explain the low expansion velocity and high density of the ring, as well as the compact ring structure and the morphology of the extended lobes. For the model to be successful, roughly 50% of the red supergiant wind mass is collimated with 10^deg^ from the equatorial plane. Such a strong collimation may indicate that the supernova progenitor had a binary companion. Finally, we discuss the implications of the hydrodynamic models in the context of the observed radio emission and high-resolution optical observations. Subject headings- circumstellar matter - hydrodynamics - stars: individual (SN 1987A) - supernovae : individual (SN 1987A)