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 ( R-T ) 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 ( \Delta R / R \la 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 disk expected to reside at the base of the R-T 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 H \alpha and in thermal-infrared ( IR ) emission ; \sim 190 K dust dominates the 8–20 \mu m luminosity ( but contains only 10 ^ { -5 } M _ { \odot } of dust ) .
Cooler ( \sim 85 K ) dust resides in the equatorial ring itself ( and has a dust mass of at least 5 \times 10 ^ { -3 } M _ { \odot } ) .
Diffuse emission extends inward to \sim 1″ from the central star , where a paucity of H \alpha and IR emission suggests an inner hole excavated by the B-supergiant wind .
We propose that diffuse emission inside the ring arises from an ionised 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 hypothesised to reside around Sk - 69°202 that was never directly detected .
If this interpretation is correct , it would suggest that photoionisation 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 .