2006ApJ...646L..61G

We explore the dynamics of ionization-bounded planetary nebulae after the termination of the fast stellar wind. When the stellar wind becomes negligible, the hot, shocked bubble depressurizes, and the thermal pressure of the photoionized region, at the inner edge of the swept-up shell, becomes dominant. At this stage the shell tends to fragment, creating clumps with comet-like tails and long, photoionized trails in between, while the photoionized material expands back toward the central stars as a rarefaction wave. Once the photoionized gas fills the inner cavity, it develops a kinematical pattern of increasing velocity from the center outward, with a typical range of velocities starting from the systemic velocity to ∼50 km.s^–1 at the edges. The Helix Nebula is a clear example of a planetary nebula at this late evolutionary stage.