Interacting winds and the shaping of planetary nebulae

Using two-dimensional hydrodynamics, the problem of shaping of planetary nebulae by the interacting winds model is studied. The shock structure is obtained and it is shown that interacting winds are capable of producing morphologies similar to the ones observed in planetary nebulae, provided that a density contrast exists between the equatorial and polar directions. This confirms recent suggestions by Balick. Mechanisms that can produce the required density contrast are discussed and it is shown that binary central stars provide the most natural mechanism, especially via common envelope evolution. It is not yet entirely clear whether stellar rotation is sufficient to generate the required contrast in the case of single central stars.