The Kinematics of Point-Symmetric Planetary Nebulae: Observational Evidence of Precessing Outflows

The discovery of collimated outflows associated to point-symmetric features in Planetary Nebulae has proliferated in recent years. The systematic variation of radial velocity that many of them show strongly suggests a uniform rotation or precession of the ejection direction. Although several physical processes have been invoked, the formation mechanism of precessing collimated outflows in PNe is currently an intriguing but unresolved problem.