And in the Darkness Bind Them: Equatorial Rings, B[e] Supergiants, and the Waists of Bipolar Nebulae

Abstract

We report the discovery of two new circumstellar ring nebulae in the western Carina Nebula. The brighter object, SBW1, resembles a lidless staring eye and encircles a B1.5 Iab supergiant. Its size is identical to the inner ring around SN1987A, but SBW1's low N abundance indicates that the star didn't pass through a RSG phase. The fainter object, SBW2, is a more distorted ring, is N-rich, and has a central star that seems to be invisible. We discuss these two new nebulae in context with rings around SN1987A, Sher25, HD168625, RY Scuti, WeBo1, SuWt2, and others. The ring bearers fall into two groups: Five rings surround hot supergiants, and all except for the one known binary are carbon copies of the ring around SN1987A. We propose a link between these rings and B[e] supergiants, where the rings derive from the same material in an earlier B[e] phase. 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. We estimate that there may be several thousand more dark rings in the Galaxy, but we are scarcely aware of their existence due to selection effects. The lower-mass objects might be the equatorial density enhancements often invoked to bind the waists of bipolar PNe.