Introduction

Abell 30 and Abell 78 are the best-known members of a small but important class of planetary nebulae (PNe) which are characterized by H-poor, dusty ejecta. Other members of this group include Abell 58 (V605 Aql), IRAS 18333-2357 (in the globular cluster M22) and IRAS 15154-5258. In these objects the H-poor material is surrounded by an outer envelope of normal composition (except for IRAS 18333-2357, where the ram pressure of the ISM would have stripped off the outer envelope: Borkowski et al. 1993a). Clearly, a secondary ejection of highly processed material has occurred after the loss of the hydrogen envelope of the AGB progenitor. A detailed interpretation was put forward by Iben et al. (1983), who proposed a final helium shell flash after nearly all of the H-rich envelope had been expelled.

The H-poor PNe are important because the composition of the ejecta opens a window upon the final phase of AGB nucleosynthesis and dredge-up, and also because the high dust to gas ratio lets us study the physics of dusty plasmas (e.g., gas heating by photoelectrons from grains: Borkowski & Harrington 1991). Here, however, we wish to point out that at least two of these objects also provide an exceptional opportunity to study mass-loaded flows. Mass-loading occurs when a tenuous, fast wind, as it streams around dense, slow-moving knots, entrains and mixes with bits of the dense material.