Stellar Evolution from AGB to Planetary Nebulae

Planetary nebulae are formed by an interacting winds process where the remnant of the AGB wind is compressed and accelerated by a later-developed fast wind from the central star. One-dimensional dynamical models have successfully explained the multi-shell (bubble, shell, crown, haloes) structures and the kinematics of planetary nebulae. However, the origin of the diverse asymmetric morphology of planetary nebulae is still not understood. Recent observations in the visible, infrared, and the submillimeter have suggested that the AGB mass loss becomes aspherical in the very late stages, forming an expanding torus around the star. A fast, highly collimated wind then emerges in the polar directions and carves out a cavity in the AGB envelope to form a bipolar nebula. Newly discovered structures such as concentric arcs, 2-D rings, multiple lobes, and point-symmetric structures suggest that both the slow and fast winds may have temporal and directional variations, and precession can play a role in the shaping of planetary nebulae. In this paper, we review the latest observations of planetary nebulae and proto-planetary nebulae and discuss the various physical mechanisms (rotation, binary, magnetic field, etc) that could lead to the observed morphologies.