Space Velocity and Time Span of Jets in Planetary Nebulae

Abstract

Fast highly-collimated outflows including bipolar knots, jet-like features, and point-symmetric filaments or string of knots are common in planetary nebulae (PNe). These features, generally named as jets, are thought to play an active role in the nebular shaping immediately before or at the same time that 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 jets in PNe cannot be determined because the inclination anglewith 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: a significant fraction ($\simeq$70%) have space velocities below 100 km s$^{-1}$, whereas only $\simeq$30% have larger velocities. Many 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 PN. The time span before a jet dissolves is found to be generally shorter than 2,500 yrs. Since most jets are found in young PNe of similar (1,000-3,000 yrs) age, it can be concluded that jets are mostly coeval with their PNe.