Planetary nebulas: Axisymmetric models

The methods of two-dimensional numerical gas dynamics have been used to investigate the evolution of the shells of planetary nebulas. The basic parameters of the model are the asymmetry and rate of matter outflow in the phase of intense mass loss (superwind). The rate of mass loss is 6 x 10-5 MassSun/yr, and the duration of the superwind phase is ∼ 103 yr. Some two-shell objects can be explained by a single ejection of matter if allowance is made for projection effects. The appearance of a planetary nebula is determined more strongly by asymmetry of the velocity of the lost matter than it is by asymmetry of its density. A critical parameter is the outflow velocity; if it is below 10 km/sec, the asymmetry of the initial mass loss may be forgotten. During the dynamical evolution, the layer of neutral gas may break up into clumps. 27 refs., 6 figs., 1 tab