On the formation of ansae in planetary nebulae

Formation mechanisms for the two optical bright knots in planetary nebulae, "ansae," are investigated. Under the assumption that the magnetic field inhibits heat conduction and showing that radiative cooling of the shocked fast wind is inefficient, we have performed adiabatic two-dimensional numerical simulations of the interacting winds flow at early stages of the planetary nebulae evolution. We find that no real focusing of the shocked fast wind material toward the symmetry axis occurs, and thus the interaction of winds by itself will not form the ansae. We then suggest and discuss a model in which jets from the central star form the ansae. These jets are presumably being formed during the few hundred years in the period between the end of the slow wind and the beginning of the fast wind. We show that a companion to the progenitor red giant can lead to the degree of asymmetry observed in elliptical planetary nebulae.