The ring nebula around the blue supergiant SBW1: pre-explosion snapshot of an SN 1987A twin★†

Abstract

SBW1 is a B supergiant surrounded by a ring nebula that is a nearby twin of SN 1987A. We present images and spectra of SBW1 obtained with HST, Spitzer, and Gemini South. HST images of SBW1 do not exhibit long Rayleigh-Taylor fingers, which are presumed to cause the hotspots in the SN1987A ring, but instead show a geometrically thin clumpy ring. The radial mass distribution and size scales of inhomogeneities in SBW1's ring closely resemble those in the SN1987A ring, but the more complete disk expected to reside at the base of the RT fingers is absent. This structure may explain why portions of the SN1987A ring between the hotspots have not yet brightened after more than 15 years. The model we suggest does not require a fast wind colliding with a previous red supergiant wind. More surprisingly, images of SBW1 also reveal diffuse emission filling the interior of the ring in H-alpha and thermal-IR emission; 190K dust dominates the 8-20 micron luminosity (but contains only 1e-5Msun). Cooler (85K dust resides in the equatorial ring itself (dust mass of 5e-3Msun). Diffuse emission extends inward to 1 arcsecond from the central star, where a paucity of emission suggests an inner hole excavated by the wind. We propose that diffuse emission inside the ring arises from an ionized flow of material photoevaporated from the dense ring, and it prevents the supergiant wind from advancing in the equator. This inner emission could correspond to a structure hypothesized to reside around SN1987A that was never directly detected. We 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 SN1987A. abridged.