Bipolar and Multipolar Jets in Protoplanetary and Planetary Nebulae

One of the most exciting challenges facing theories of post-main sequence evolution today is to understand how Asymptotic Giant Branch (AGB) stars and their round circumstellar envelopes (CSEs) transform themselves into planetary nebulae (PNe) with their dazzling variety of aspherical morphologies. The most succesful model for shaping PNe—the “generalised interacting-stellar-winds” model, in which a fast (> 1000 km s) spherical stellar wind interacts with an equatorially-dense AGB CSE to produce an axisymmetric PN—now appears inadequate as a result of new data from the Hubble Space Telescope (HST ). This paper presents new HST images of proto-planetary and planetary nebulae with collimated outflows. The outflows appear to be quite often multipolar in nature, indicating episodic changes in the axis of a bipolar jet-like outflow or the operation of multiple collimated outflows with different orientations. In particular, our serendipitous discovery of a very highly-collimated, extended jet in the planetary nebula Hen 2-90 and its amazing morphological similarity to low-mass YSOs is the strongest evidence yet for a common physical mechanism for collimated outflows in protostars and evolved stars. We briefly summarise current theoretical hypotheses which may lead to a better understanding of the nature and origin of these outflows.