FLIERS AND OTHER MICROSTRUCTURES IN PLANETARY-NEBULAE - IV - IMAGES OF ELLIPTIC PNS FROM THE HUBBLE-SPACE-TELESCOPE

We report new results from high spatial resolution Wide Field Planetar y Camera 2 imaging studies of ''FLIERs'' and other microstructures in the planetary nebulae NGC 3242, 6826, 7009, and 7662. Most FLIERs have head-tail morphologies, with the tails pointing outward from the nucl eus. Ionization gradients that decrease with distance from the nebular center are ubiquitous. These are consistent with an ionization front in neutral knots of density approximate to 10(4) cm(-3). Can neutral k nots account for the properties of FLIERs? We compare two broad classe s of possible explanations for FLIERs with the new images: high-speed bullets ramming through the shells of planetary nebulae, and photoevap orated gas swept by winds into head-tail shapes. Both classes of model s fail basic consistency tests. Hence an entirely new conceptual parad igm is needed to account for the phenomenology of FLIERs.