2001ApJ...556..823S

We present an alternative model for the formation of fast low-ionization emission regions (FLIERs) in planetary nebulae that is able to account for many of their attendant characteristics and to circumvent the problems of the collimation/formation mechanisms found in previous studies. In this model, the section of the stellar wind flowing along the symmetry axis carries less mechanical momentum than that at higher latitudes and temporarily develops a concave or inverted shock geometry. The shocked ambient material is thus refracted toward the symmetry axis, instead of away from it, and accumulates in the concave section. The reverse is true for the outflowing stellar wind, which in the reverse shock is refracted away from the axis. It surrounds the stagnation region of the bow shock and confines the trapped ambient gas. The latter has time to cool and is then compressed into a dense ``stagnation knot'' or ``stagnation jet.'' In the presence of a variable stellar wind these features may eventually overrun the expanding nebular shell and appear as detached FLIERs. We present representative two- and three-dimensional hydrodynamic simulations of the formation and early evolution of stagnation knots and jets and compare their dynamical properties with those of FLIERs in planetary nebulae.