Last year we reported the discovery of a ring nebula called SBW1 around a blue supergiant star in our Galaxy. In almost every respect, it is a true "twin" of the equatorial ring nebula around SN 1987A: it has an identical physical radius of 0.2pc, a similar expansion speed and age, it is located in a massive HII region, the central star is an early B supergiant with the same luminosity as
... [Show full abstract] SN1987A's progenitor, and the ring's structure in our ground-based H-alpha images looks almost identical to early HST images of SN1987A's ring. The detailed density structure of SN1987A's ring on scales smaller than the limiting resolution of HST has become a pressing question, because the forward shock of the supernova is now colliding with that ring, causing it to brighten by 3 orders of magnitude and giving rise to a series of "hotspots" around the ring. HST/WFC3 images of SBW1 will provide a snapshot of an SN1987A-like ring before the supernova explodes, and will provide a detailed view of the important density inhomogeneities in the ring with a physical spatial resolution 10 times better than HST images of SN1987A {because SBW1 is 10 times closer to us}. STIS spectra will allow us to directly measure the radial density structure of the ring. Both the overall radial density profile and the detailed structures of the clumps that give rise to the "hotspots" are critical factors in modeling the rapid evolution of SN1987A, and our proposed study of SBW1 will provide extremely valuable input for those models.;
