Abstract
We present an alternative model for the formation of fast low-ionization emission regions (FLIERs) in planetary nebulae. The model is able to account for many of their attendant characteristics and circumvent the problems related to the collimation/formation mechanisms found in previous studies. In this model, a concave bow-shock structure is formed as a result of a reduced momentum flow along the symmetry axis of a stellar wind. FLIERS are formed from the shocked ambient medium. Since in the concave region of the bow-shock the ambient material can not flow away from the symmetry axis, it is compressed into a dense knot of jet-like feature (which we call ``stagnation knot'' or ``stagnation jet''). In the presence of a variable stellar wind these knots eventually overrun the expanding nebular shell and appear as detached FLIERS. We present two-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.
