Space Velocity and Time Span of Jets in Planetary Nebulae

Fast highly collimated outflows, including bipolar knots, jetlike features, and point-symmetric filaments or strings of knots, are common in planetary nebulae (PNe). These features, generally known as jets, are thought to play an active role in the nebular shaping immediately before or while fast stellar winds and D-type ionization fronts shock and sweep up the nebular envelope. The space velocity, radial distance from the central star, and kinematic age of the jets in PNe cannot be determined because the inclination angle with the line of sight is usually unknown. Here we have used the large number of jets already detected in PNe to derive orientation-independent properties from a statistical point of view. We find that jets in PNe can be assigned to two different populations: most (about 70%) have space velocities below 100 km s(-1), and only 30% have larger velocities. Since a significant fraction of jets move at velocities similar to that of their parent PNe and are found close to the nebular edge, we propose that these jets have been slowed down in their interaction with the nebular envelope, contributing to the expansion of their PNe. The time spans before the jets dissolve are found to be generally shorter than 2500 yr. Since most jets are found in young PNe of similar (1000-3000 yr) age, it can be concluded that jets are mostly coeval with their PNe.