A new hydrodynamic model for the formation of the Homunculus Nebula around η Carinae

Recent models of the Homunculus Nebula in eta Carinae have been unable to account for the fact that no large, extended disk-like distribution is seen around the nebula which could have collimated the bipolar lobes. We have constructed an interacting winds scenario that accounts for the geometric and kinematic properties of the Homunculus as seen in recent observations using the Hubble Space Telescope. Winds from a giant eruption starting in 1840 sweep into a small (10(14) cm), dense (similar to 10(14) cm(-3)), two-solar-mass, near-nuclear toroidal ring. The external medium is assumed to have a uniform density. 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 few years. Subsequent weaker outflows ram into the outburst gas, leading to the formation of surface instabilities and wrinkles. Models with cooling form essentially ballistic flows (that is, a pair of cones each with a spherical base) whose lateral edges are wrinkled due to shear instabilities. The fragmentation of the dense ring in the radiative models may help to explain the thin, radial filamentary structure that is seen in the equatorial region of the Homunculus. Adiabatic models, which contain gas at high temperatures within the interior, explode through the nascent cones into the confining gas before the dense collar is destroyed. A pair of spherical lobes form, whose walls get corrugated by shearing instabilities. In a few hundred years these lobes morph into one large single balloon.