THE FORMATION OF POINT-SYMMETRICAL NEBULAE

A number of the extended planetary nebulae show point-symmetric morpho logy, and it has been suggested that such a morphology is the conseque nce of the episodic ejection of material in a two-sided jet. Here we s how that the accretion disk, previously postulated to be formed in the final stages of common envelope evolution giving rise to the planetar y nebulae, is unstable to a radiation-induced self-warping instability discovered by Pringle. We show further that the timescales on which t he warping instability and concomitant induced precession occur are sh ort enough to be relevant to the planetary nebula ejection process.