Multiple Shell Planetary Nebulae: Evolutionary Paths And Observed Configurations

Recent observations of multiple shell planetary nebulae confirm the existing subdivision into two morphological types: attached halo and detached halo. The multiple shell phenomenon appears in about twenty-five percent of round and elliptical planetary nebulae of the Instituto de Astrofisica de Canarias morphological survey. The analysis of these nebulae together with that of their central stars lead to the interpretation that detached halo nebulae are formed through discontinuous mass loss at the Asymptotic Giant Branch tip. The attached halo nebulae, instead, are probably produced via dynamical nebular shaping.