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R. Sahai (Jet Propulsion Laboratory/ Caltech)
One of the most exciting challenge facing theories of post-main sequence evolution today is to understand how Asymptotic Giant Branch (AGB) stars transform themselves into aspherical planetary nebulae (PNe). PNe form from intermediate mass stars (1-8 M\sun) after a phase of intense mass-loss on the AGB, when the central star temperature exceeds 30,000K, and stellar UV ionises the ejected gas. PNe show bright rims and well defined shell-like structures, and a dazzling variety of morphologies, which include complicated axisymmetric and point-symmetric. In contrast, the extended circumstellar envelopes (CSEs) of most AGB stars appear largely spherically symmetric.
There is no consensus yet on the physical mechanism(s) responsible for producing the dramatic transition in the geometry of the ejected matter. However, there is increasing observational evidence that fast collimated outflows, operating during the late-AGB or proto-planetary phases, play a crucial role in this transition. The most succesful model for shaping PNe - the ``generalised interacting-stellar-winds'' model, in which a fast (>1000~km~s-1) spherical stellar wind interacts with an equatorially-dense AGB CSE to produce an axisymmetric PN - no longer appears adequate.
This paper reviews the observations, with special emphasis on recent results from HST. An important new result of these data is that the outflows are quite often multipolar in nature, indicating episodic changes in the axis of an bipolar outflow or the operation of multiple collimated outflows with different orientations. We briefly summarise current theoretical hypotheses which may lead to a better understanding of the nature and origin of these outflows. We conclude with describing the discovery of a very highly-collimated, extended jet in a planetary nebula - its amazing morphological similarity to a low-mass YSO is the strongest evidence yet for a commom physical mechanism for collimated outflows in protostars and evolved stars.
This work is funded by NASA under a Long Term Space Astrophysics grant.
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The author(s) of this abstract have provided an email address for comments about the abstract: sahai@grandpa.jpl.nasa.gov