We summarize the analysis of a uniform set of both previously-known and newly-discovered scattered-light echoes , detected within 30 ^ { \prime \prime } of SN 1987A in ten years of optical imaging , and with which we have constructed the most complete three-dimensional model of the progenitor ’ s circumstellar environment .
Surrounding the SN is a richly-structured bipolar nebula .
An outer , double-lobed “ peanut , ” which we believe is the contact discontinuity between the red supergiant and main sequence winds , is a prolate shell extending 28 ly along the poles and 11 ly near the equator .
Napoleon ’ s Hat , previously believed to be an independent structure , is the waist of this peanut , which is pinched to a radius of 6 ly .
Interior , the innermost circumstellar material lies along a cylindrical hourglass , 1 ly in radius and 4 ly long , which connects to the peanut by a thick equatorial disk .
The nebulae are inclined 41°south and 8°east of the line of sight , slightly elliptical in cross section , and marginally offset west of the SN .
The 3-D geometry of the three circumstellar rings is studied , suggesting the northern and southern rings are located 1.3 and 1.0 ly from the SN , while the equatorial ring is elliptical ( b / a \lesssim 0.98 ) , and spatially offset in the same direction as the hourglass .
Dust-scattering models of the observed echo fluxes suggest that between the hourglass and bipolar lobes : the gas density drops from 1–3 cm ^ { -3 } to \gtrsim 0.03 cm ^ { -3 } ; the maximum dust-grain size increases from \sim 0.2 \mu m to 2 \mu m ; and the silicate : carbonaceous dust ratio decreases .
The nebulae have a total mass of \sim 1.7 M _ { \sun } , yielding a red-supergiant mass loss around 5 \times 10 ^ { -6 } M _ { \sun } yr ^ { -1 } .
We compare these results to current formation models , and find that no model has successfully reproduced this system .
However , our results suggest a heuristic evolutionary sequence in which the progenitor evolves through two “ blue-loops , ” perhaps accompanied by a close binary companion .