2013MNRAS.429.1324S

SBW1 is a B-type supergiant surrounded by a ring nebula that is a nearby twin of SN 1987A's progenitor and its circumstellar ring. We present images and spectra of SBW1 obtained with the Hubble Space Telescope (HST), the Spitzer Space Telescope and Gemini South. HST images of SBW1 do not exhibit long Rayleigh-Taylor (RT) fingers, which are presumed to cause the `hotspots' in the SN 1987A ring when impacted by the blast wave, but instead show a geometrically thin (ΔR/R ≲ 0.05) clumpy ring. The radial mass distribution and size scales of inhomogeneities in SBW1's ring closely resemble those in the SN 1987A ring, but the more complete disc expected to reside at the base of the RT fingers is absent in SBW1. This structure may explain why portions of the SN 1987A ring between the hotspots have not yet brightened, more than 15 years after the first hotspots appeared. The model we suggest does not require a fast wind colliding with a previous red supergiant wind, because a slowly expanding equatorial ring may be ejected by a rotating blue supergiant star or in a close binary system. More surprisingly, high-resolution images of SBW1 also reveal diffuse emission filling the interior of the ring seen in Hα and in thermal-infrared (IR) emission; ∼ 190K dust dominates the 8-20µm luminosity (but contains only 10-5M☉ of dust). Cooler ( ∼ 85K) dust resides in the equatorial ring itself (and has a dust mass of at least 5x10-3M☉). Diffuse emission extends inward to ∼ 1arcsec from the central star, where a paucity of Hα and IR emission suggests an inner hole excavated by the B-supergiant wind. We propose that diffuse emission inside the ring arises from an ionized flow of material photoevaporated from the dense ring, and its pressure prevents the B-supergiant wind from advancing in the equatorial plane. This inner emission could correspond to a structure hypothesized to reside around Sk-69°202 that was never directly detected. If this interpretation is correct, it would suggest that photoionization can play an important dynamical role in shaping the ring nebula, and we speculate that this might help explain the origin of the polar rings around SN 1987A. In effect, the photoevaporative flow shields the outer bipolar nebula at low latitudes, whereas the blue supergiant wind expands freely out the poles and clears away the polar caps of the nebula; the polar rings reside at the intersection of these two zones.