Why a Single-Star Model Cannot Explain the Bipolar Nebula of Eta Carinae

I examine the angular momentum evolution during the 1837-1856 Great Eruption of the massive star η Carinae. I find that the new estimate of the mass blown during that eruption implies that the envelope of η Car substantially spun-down during the 20 years eruption. Single-star models, most of which require the envelope to rotate close to the break-up velocity, cannot account for the bipolar nebula−the Homunculus−formed from matter expelled in that eruption. The kinetic energy and momentum of the Homunculus further constrains single-star models. I discuss how η Car can fit into a unified model for the formation of bipolar lobes where two oppositely ejected jets inflate two lobes (or bubbles). These jets are blown by an accretion disk, which requires stellar companions in the case of bipolar nebulae around stellar objects.