And in the darkness bind them: Equatorial rings, B[e] supergiants, and the waists of Bipolar Nebulae¹

We report the discovery of two new circumstellar ring nebulae in the western Carina Nebula, and we discuss their significance in stellar evolution. The brighter of the two new objects, SBW 1, resembles a lidless staring eye and encircles a B1.5 lab supergiant. Although seen in Carina, its luminosity class and radial velocity imply a larger distance of ∼7 kpc in the far Carina arm. At that distance its size and shape are nearly identical to the equatorial ring around SN 1987A, but SBW 1 's low N abundance indicates that the ring was excreted without its star passing through a red supergiant phase. The fainter object, SBW 2, is a more distorted ring, is N-rich, and is peculiar in that its central star seems to be invisible. We discuss the implications of these two new nebulae in context with other circumstellar rings such as those around SN 1987A, Sher 25, HD 168625, RY Scuti, WeBo 1, SuWt 2, and others. The ring bearers fall into two groups: Five rings surround hot supergiants, and it is striking that all except for the one known binary are carbon copies of the ring around SN 1987A. We propose a link between these B supergiant rings and B[e] supergiants, where the large spatially resolved rings derive from the same material that would have given rise to emission lines during the earlier B[e] phase, when it was much closer to the star. The remaining four rings surround evolved intermediate-mass stars; all members of this ring fellowship are close binaries, hinting that binary interactions govern the forging of such rings. Two-thirds of our sample are found in or near giant H II regions. We estimate that there may be several thousand more dark rings in the Galaxy, but we are scarcely aware of their existence-either because they are only illuminated in precious few circumstances or because of selection effects. For intermediate-mass stars, these rings might be the preexisting equatorial density enhancements invoked to bind the waists of bipolar nebulae.