We apply recent theoretical results of gasdynamic instabilities to interpret observations of 34 planetary nebulae moving through, and interacting with, the interstellar medium. We show that the Rayleigh-Taylor instability can play an important role not only in shaping the outskirts of the nebulae but in shaping the inner regions as well, since it allows the interstellar medium to flow into the nebular inner parts by fragmenting the halo. This is nicely demonstrated, we claim, in the interacting planetary nebulae A35 and IC 4593, which have bow shocks well inside their almost spherical, but very filamentary, halos. The fragmentation by the Rayleigh-Taylor instability relaxes the need to assume nonhomogeneous mass ejection by these planetary nebulae progenitors. We also apply recent results that suggest that the interstellar medium magnetic field makes the Rayleigh-Taylor instability very efficient for nebulae close to the Galactic plane and breaks the cylindrical symmetry of Rayleigh-Taylor and Kelvin-Helmholtz instability modes. The deviation from axisymmetry in the interaction process forms what we term "Rayleigh-Taylor rolls" (or stripes), instead of "fingers" or "blobs," which form in the unmagnetized flow.
- ISM: kinematics and dynamics
- ISM: magnetic fields
- ISM: structure
- Planetary nebulae: individual (a35, IC 4593)