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J. Patrick Harrington, Department of Astronomy, University of Maryland, College Park, MD 20742, U.S.A.,
Kazimierz J. Borkowski, Department of Astronomy, University of Maryland, College Park, MD 20742, U.S.A.,
Zlatan Tsvetanov, Center for Astrophysical Sciences, Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, U.S.A.,
Robin E.S. Clegg, Royal Greenwich Observatory, Madingley Road, Cambridge CB3 OEZ, U.K.
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.
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