Interaction of planetary nebulae with the interstellar medium: theory.

We performed hydrodynamical simulations of the interaction between planetary nebulae (PNs) and the interstellar medium (ISM), in order to provide a theoretical framework for observations of interacting PNs. Our results validate a thin-shell approximation originally developed by Smith [MNRAS, 175, 419 (1976)] to calculate the expected displacement of central stars from PN centers, for a typical nebula in which the shocked ISM gas cools down to a characteristic nebular temperature of ∼10⁴ K. This simple picture breaks down for high-velocity PNs in a low density environment, where the shocked ISM cools slowly. In this case, a Rayleigh-Taylor instability develops, leading to shell fragmentation on roughly the same timescale on which the shell is decelerated. We use our results to interpret observations of two interacting nebulae of particular interest, NGC 246 and the PN in the globular cluster M22.