Why a Single‐Star Model Cannot Explain the Bipolar Nebula of η Carinae

Abstract

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 yr eruption. Single-star models, most of which require the envelope to rotate at close to the break-up velocity, cannot account for the bipolar nebula - the Homunculus - formed from matter expelled in mat eruption. The kinetic energy and momentum of the Homunculus further constrain single-star models. I discuss how η Car can fit into a unified model for the formation of bipolar lobes in which 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.