A model for the formation of high-density clumps in proto-planetary Nebulae

The detection of NaCl at large radii in the Egg Nebula, CRL 2688, requires densities of 10⁷-10⁸ cm^-3 in a thick shell of r ∼ a few × 10¹⁷ cm. To explain these results, a mechanism is needed for producing high densities at a considerable distance from the central star. In two-dimensional simulations of the interaction of the fast wind with an ambient medium, the material becomes thermally unstable. The resulting clumps can achieve the requisite conditions for NaCl excitation. We present two-dimensional models with simple physics as proof-of-principle calculations to show that the clumping behavior is robust. Clumping is a natural outcome of cooling in the colliding wind model and comparable to that inferred from observations.