The evolution of planetary nebulae. I - Structures, ionizations, and morphological sequences

An atlas of CCD pictures of fifty-one planetary nebulae (PNs) taken in the light of low-, moderate-, and high-ionization emission lines is presented. The shapes of many of the PNs can be organized into an empirical morphological sequence of round, elliptical, bipolar, and butterfly classes. Many PNs, especially the ones of high surface brightness such as NGC 2392, 3242, 6543, 6826, and 7662, show evidence for thin inclusions of anomalously low ionization in a high-ionization substrate. The morphological sequence of PNs is shown to be consistent with the precepts of the shaping of PNs by interacting winds. This consistency places the concepts of ongoing hydrodynamic shaping of PNs on firm grounds, and justifies further observations that can lead to a better understanding of the physical processes. PNs are proposed as an ideal astrophysical laboratory for studying the interactions of winds on a structured environment.