The authors present observations mapping the velocity of the gas in Hα and [N II] for the planetary nebulae NGC 40, 2392, 3242, 6543, 6826, 7009, 7354, and 7662. All of these show spatial and kinematic features suggestive of the existence of highly collimated fast outflows, generally characterized by velocities (projected onto the sky) in the range of 10 - 60 km s-1. NGC 2392 is extreme in its outflow velocities of ≡170 km s-1. To explain the observations, the authors propose a hydrodynamic mechanism that collimates the gas as it passes outward through a prolate shock front. They adopt the usual fast-wind model, but assume that the slow-wind envelope that surrounds the star is denser near its equator than near the poles. The model is discussed in detail and a comparison with the observations is presented.
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Balick et al. (1987) studied this question.