How Fast Are Collimated Outflows in Planetary Nebulae?

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 ; only a small fraction of them expand faster. The discovery of jet-like features, pairs of pointsymmetric bright knots, and ansae in PNe has proliferated in recent years (see the review by López 2000). These features are typically highlighted in the emission of low-ionization lines and have peculiar expansion velocities relative to the surrounding nebula. They are often cited as collimated high-velocity outflows, but their true space velocities are unknown because the inclination angles of the outflows are not known and therefore the observed radial velocities cannot be corrected for projection effects. It is paradoxical that some of the observed velocities are slower than the expansion velocities of the host PNe (e.g., IC 4593, Corradi et al. 1997). There exists a large number of PNe in which collimated outflows have been verified by high-dispersion spectroscopic observations. It is thus possible to undertake a statistical analysis of the distribution of the apparent expansion velocities (Vexp) and projected radial positions of these collimated outflows in PNe in order to investigate their true space velocities. We have compiled the Vexp and radial distances of collimated outflows in 33 PNe. The observed distribution of Vexp (Fig. 1) peaks at ∼ 30 km s −1 and drops sharply at higher velocities. We have compared this distribution to those expected from simple distributions of the space velocity. We find that the observed distribution is consistent with a bi-modal