Abstract
A significant fraction of planetary nebulae (PNe) show jet-like features. These features are usually interpreted as fast collimated outflows, although their true space velocities are uncertain, as their measured radial velocities cannot be properly corrected for projection effects. We have compiled the measured expansion velocities and radial distances of collimated outflows for a sample of 33 PNe and analyzed statistically their distributions. Our analysis indicates that the true space velocities of collimated outflows have a bi-modal distribution. Most of them are consistent with true expansion velocities <= 70 km s-1; only a small fraction of them expand faster.
