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Paper: Hubble Space Telescope Observations of Young Planetary Nebulae
Volume: 199, Asymmetrical Planetary Nebulae II: From Origins to Microstructures
Page: 209
Authors: Sahai, Raghvendra
Abstract: We present results from an ongoing Hubble Space Telescope Hα imaging survey of young planetary nebulae (PNe), selected on the basis of their low excitation characteristics. All objects imaged so far show highly aspherical morphology, with a majority characterised by multipolar bubbles distributed roughly point-symmetrically around the central star. In some objects, bipolar ansae or collimated radial structures are seen, indicating the presence of jets, whereas in others bright structures near the minor axes indicate the presence of disks or torii. The complexity, organization and symmetry of the above structures leads us to propose that the primary agent for shaping PNe are high-speed collimated outflows or jets which operate during the late AGB and/or early post-AGB evolutionary phase. These outflows carve out a complex imprint within an intrinsically spherical AGB circumstellar envelope (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 how the characteristics of the jets change with time.
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