The Formation of Point-symmetric Nebulae

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