The Ring Nebula Around the Blue Supergiant SBW1: Pre-Explosion Snapshot of a SN 1987A Twin

TL;DR

This study presents detailed imaging and spectral analysis of SBW1, a blue supergiant with a ring nebula similar to SN 1987A, revealing structural features and ionization effects that shed light on pre-supernova conditions.

Contribution

It provides the first detailed comparison of SBW1's ring structure and internal emission with SN 1987A, proposing a new model involving photoevaporation and ionization effects without requiring a fast wind collision.

Findings

SBW1's ring lacks Rayleigh-Taylor fingers but has similar inhomogeneity scales to SN 1987A.

Diffuse emission inside the ring is dominated by 190K dust, with an inner hole likely created by wind activity.

Photoionization and photoevaporation may significantly influence nebula shaping and explain features of SN 1987A.

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.