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R. Sahai, J. Trauger (JPL/ Caltech)
We report on new results from our continuing H\alpha imaging survey of young planetary nebulae (PNe) using the Wide Field and Planetary Camera 2 onboard the Hubble Space Telescope. The primary objective of this survey is to address the long-standing puzzle of how the round circumstellar envelopes (CSEs) of Asymptotic Giant Branch (AGB) stars evolve into the dazzling variety of aspherical PNe observed. A set of PNe selected solely on the basis of their low excitation characteristics, were imaged during 1996. These PNe showed highly aspherical morphology, with a majority characterised by multipolar bubbles and point-symmetric structure. The data, difficult to explain under the existing ``Generalised Interacting Stellar Winds (GISW)" paradigm (Balick, AJ, 1987, 94, 671), have led us (Sahai & Trauger 1998, AJ, 166, 1357) to hypothesise that fast collimated outflows with changing directionality are the primary agent for the formation and shaping of PNe. These outflows carve out a complex imprint within an intrinsically spherical AGB CSE. Subsequent expansion of a hot, tenuous stellar wind from the post-AGB star inside the imprinted AGB CSE then produces the observed PN, whose shape and structure depend in detail on the history of the fast outflows.
We present images of many new PNe observed in 1999 as part of our HST/SNAPshot program. As before, none of the observed PNe are round, and multi/bi--polar morphologies abound, with substantial evidence for point-symmetry in many objects. Comparison of the morphologies of our young PNe sample with more evolved PNe populations should lead to new constraints on models for the production of jet-like outflows in late-AGB/post-AGB objects and subsequent nebular evolution. Our HST survey underscores the critical need for carrying out extensive theoretical modelling of the interaction of fast-collimated winds with AGB CSEs.
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