Jets in planetary nebulae

It is assumed that the acceleration and collimation mechanisms of jets are the same in all the classes of astrophysical objects which are observed to produce jets. These classes now include such objects as: active galactic nuclei, young stellar objects, massive x-ray binaries, black hole x-ray transients, symbiotic systems, planetary nebulae, supersoft x-ray sources, and possibly recurrent novae. On the basis of this assumption, an attempt is made, to identify the necessary ingredients for the acceleration and collimation mechanism. It is argued that: (i) jets are produced mainly at the center of accretion disks, and are accelerated and collimated hydromagnetically, (ii) the production of powerful jets requires a hot corona or access to an additional energy source associated with the central object. Tentative explanations for the presence of jets in some classes of objects and absence in others are given. Some critical observations that can test the ideas presented in this paper are suggested.