Jets and Tori in Proto-Planetary Nebulae

We investigate the time sequence for the appearance of jets and molecular tori in the transition of stars from the Asymptotic Giant Branch to the planetary nebula phase. Jets and tori are prominent features of this evolution, but their origins are uncertain. Using optical and millimeter line kinematics, we determine the ejection history in a sample of well-observed cases. We find that jets and tori develop nearly simultaneously. We also find evidence that jets typically appear slightly later than tori, with a lag time of a few hundred years. These characteristics provide strong evidence that jets and tori are physically related, and they set new constraints on theories of jet formation. The ejection of a discrete torus followed by jets on a short time scale favors the class of models in which a companion interacts with the central star. Models with long time scales, or with jets followed by a torus, are ruled out. Subject headings: circumstellar matter — planetary nebulae: general — stars: AGB and post-AGB — stars: mass loss