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The formation of planetary nebulae can be discussed in terms of an interacting stellar winds model, in which the slow wind from the progenitor star is later overtaken by the fast wind from the central star. As a result of the interaction the inner region of the slow wind is compressed into a dense shell. We investigate the consequences for the model of a red-giant wind in which the mass-loss rate is enhanced towards the equator. The resulting shell is elongated in the direction of the poles. Our treatment is simplified, in that we neglect the finite temperature of the compressed shell. Consequently, the cusps which we sometimes find at the equator are, in reality, regions of high density.
Kahn et al. (Fri,) studied this question.