On the formation of the Homunculus nebula around η Carinae

We have constructed an interacting winds scenario to account far the geometric and kinematic properties of the Homunculus in eta Carinae as seen in recent Hubble Space Telescope observations. Winds from a giant eruption in 1840-1860 sweep into a small (10(14) cm), dense (similar to 10(14) cm(-3)), 2 M-., near-nuclear toroidal ring. The external medium is uniform at similar to 2000 particles cm(-3). The ring is all but destroyed by the winds in the eruption. Even so, it manages to provide a good deal of collimation to the mass ejected in the first 20 years. Subsequent weaker outflows ram into the outburst gas and initiate surface instabilities and wrinkles. Unlike earlier models, ours is in accordance with the observation that no large, extended disklike distribution is seen around the nebula that could have collimated the bipolar lobes. Models with cooling form essentially ballistic hows (that is, a pair of cones each with a spherical base) whose lateral edges become wrinkled by shear instabilities. A new aspect of the radiative models is the fragmentation of the dense ring, which may help to explain the thin, radial filamentary structure that is seen in the equatorial region of the Homunculus. Adiabatic models become very hot quickly and explode through the nascent cones into the confining gas before the dense collar is destroyed. A pair of spherical lobes form. After 150 years the lobe walls are corrugated by shearing instabilities. These lobes morph into a large, single balloon after about another 300 years.