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Session 15 - Planetary Nebulae and White Dwarfs.
Display session, Wednesday, January 07
Exhibit Hall,
We have carried out simulations of PNe within the general framework of the Interacting Stellar Winds Models, wherein the fast wind from a central star sweeps up the slow wind emitted in a previous epoch. Our two-dimensional simulations take into account the evolution of the velocity and mass loss rate of the fast wind. This leads to considerably more structure on smaller scales than was seen in those nebulae where the fast wind velocity is held constant. The nebula evolves through an initial momentum-conserving phase before entering the more commonly encountered energy-conserving stage. Both stages are prone to instabilities, which may be the precursor of knots, filaments, and other micro-structures that are now commonly seen in HST images of PNe. In particular we note the occurrence of the Non-Linear Thin Shell Instability in the early stages, and the formation of Rayleigh-Taylor filaments in the energy-conserving stage. The growth of small-scale structure in the momentum-conserving stage is sensitive to the ratio of the initial wind momenta; the lifetime of the nebula in this stage depends also on the evolution of the wind properties. The overall size scale of the system is determined mainly by the evolution of the fast wind properties. If the evolution is not taken into account when computing kinematic ages then the ages may be underestimated.
Some of the simulations presented here were carried out on the C90 at the Pittsburgh Supercomputing Center and the SGI Power Challenge at NCSA. This work was supported by NSF grant AST9417112, and by the award of a Theodore Dunham Jr. grant for 1996 from the Fund for Astrophysical Research.
