THE IMPACT OF SN 1987A WITH ITS CIRCUMSTELLAR RING

The envelope of SN 1987A will strike its circumstellar ring in 12 +/- 3 yr after explosion (A.D. 1999 +/- 3) the exact time depending weakly on the uncertain density of diffuse gas between the supernova and the ring, The impact will drive a radiative shock into the ring with velocity approximately 200-400 km s-1. The shocked ring will become a bright optical and ultraviolet emission-line source. A bright arc will suddenly appear at the near side of the ring and grow into an entire ring about 11 months later. The luminosities of the brightest lines, Halpha, Hbeta, N V lambdalambda1238, 1242, and O VI lambdalambda1032, 1038 will rise rapidly to approximately 10(36)-10(37) ergs s-1 and remain bright for several years after impact. The emission lines from the shocked ring will have FWHM approximately 300-600 km s-1 and complex profiles that will depend on position and will be sensitive to the details of the density distribution of gas in the ring. Strong EUV radiation from the shock will photoionize the unshocked ring, causing emission of narrow (FWHM approximately 15 km s-1) Halpha, Hbeta, and [O III] lambdalambda4959, 5007 lines with luminosities approximately 10(35) ergs s-1. The EUV radiation will probably cause the nebulosity beyond the ring to become visible again. The EUV radiation may also illuminate the unshocked outer supernova envelope, causing visible emission of broad (FWHM approximately 10(4) km s-1) Halpha and Hbeta lines.