INSTABILITIES IN MOVING PLANETARY-NEBULAE

We apply recent theoretical results of gasdynamic instabilities to int erpret observations of 34 planetary nebulae moving through, and intera cting with, the interstellar medium. We show that the Rayleigh-Taylor instability can play an important role not only in shaping the outskir ts of the nebulae but in shaping the inner regions as well, since it a llows the interstellar medium to flow into the nebular inner parts by fragmenting the halo. This is nicely demonstrated, we claim, in the in teracting planetary nebulae A35 and IC 4593, which have bow shocks wel l inside their almost spherical, but very filamentary, halos. The frag mentation by the Rayleigh-Taylor instability relaxes the need to assum e nonhomogeneous mass ejection by these planetary nebulae progenitors. We also apply recent results that suggest that the interstellar mediu m magnetic field makes the Rayleigh-Taylor instability very efficient for nebulae close to the Galactic plane and breaks the cylindrical sym metry of Rayleigh-Taylor and Kelvin-Helmholtz instability modes. The d eviation from axisymmetry in the interaction process forms what we ter m ''Rayleigh-Taylor rolls'' (or stripes), instead of ''fingers'' or '' blobs,'' which form in the unmagnetized flow.