Structure and evolution of planetary nebulae

The dynamical evolution of a model of an ideal planetary nebula is studied. In this picture, the H I shell, which surrounds an ionized sphere during the early stages in the evolution of a nebula, is accelerated outward by the thermal and dynamical pressure at its inner boundary or ionization front. The ionization front becomes dynamically unstable when the nebula has expanded and rarefied to the point where recombinations and ionizations become too slow to suppress positive (away from the central star) and negative (toward the central star) displacements, respectively, of the ionization front. In that case, the development of instability at the ionization front is similar to the development of the Rayleigh-Taylor instability at the interface of superposed, incompressible fluids of differing density. Once the ionization front distorts and spikes of dense, neutral gas begin to penetrate the ionized sphere, shearing forces at the surfaces of the spikes are induced by velocity differences between the spikes and the adjacent ionized gases. Under the influence of these shearing forces, the spikes distort and form dense, globular condensations. Of the order of l0 globules, each having a mass of about 2 x l0 g, form when the H I shell fragments. Over one-half of the nebular mass is in the form of condensations at this time. After the H I shell fragments into globular condensations, the globules erode under the influence of ionizing radiation from the central star. However, the rate of production of ionizing radiation by the evolving central star decreases so rapidly that a typical condensation retains about 30 percent of its original mass. Therefore, a planetary nebula may inject l0 condensations, each having a mass of about l0 g, into the interstellar medium. After fragmentation of the H I shell, ionized gas flows outward through spaces between the condensations. Part of this gas flows into regions that are shielded by the globules against direct, ionizing radiation from the central star. In the shadowed regions, the level of ionization is sufficiently low that emission in forbidden lines of singly ionized oxygen and singly ionized nitrogen predominates. In particular, the model predicts the radial, filamentary structure that is observed when NGC 7293 is photographed in the light of Ha and [N ii] (A6548 + A6584). The model also predicts that practically all [0 I] A6300 radiation arises in the shadowed zones. Photographs of planetary nebula taken in the light of [0 I] A6300 do indeed show very marked striation. Subject headings: gas dynamics - planetary nebulae