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R. Sahai (JPL, Caltech), D. Le Mignant (W.M Keck Observatory), C. Sanchez Contreras (Caltech), M. Stute (JPL, Caltech), M. Morris (UCLA)
The shaping of planetary nebulae (PNs) is probably the most exciting yet least understood problem in the late evolution of intermediate mass stars. PNs evolve from the envelopes of AGB stars via a supposedly short (~1000 yr) pre-planetary nebula (PPN) phase. HST imaging of PPNs and PNs has shown the widespread presence of diverse bipolar and multipolar morphologies. In 1998, in a radical departure from the long-standing theoretical paradigm for PN formation, Sahai & Trauger proposed that as most stars evolve off the AGB, they drive collimated fast winds that sweep up and shock the AGB circumstellar envelope, producing the observed dramatic changes in circumstellar geometry and kinematics from the AGB to the PN phase.
The search for these collimated jets has proved to be rather elusive, partly because these are most likely episodic and operate only for a few x 100 years in the early PPN phase. During this phase, much of the circumstellar environment, including the central dusty waist of these nebulae, is optically-thick at visible wavelengths. We are therefore carrying out a program of observing PPNs with the LGSAO system on Keck II at near-infrared (1.1-4.7 micron) wavelengths. Our very first attempt met with remarkable success -- observations of the bipolar young PPN, IRAS16342-3814, revealed a remarkable corkscrew-shaped structure apparently etched into the lobe walls -- direct signature of an underlying precessing jet. Here we present results from new high-resolution (55 mas at 2 micron) observations of a small sample of PPNs with the LGSAO system. As in their HST images, our objects display bipolar/multipolar morphologies, but in addition, the bubble-like ``wind-swept" structure of the lobes is clearly revealed. Furthermore, the dusty waists appear much thinner geometrically than in the HST images, but surprisingly, in some PPNs, the central stars still remain obscured, with important implications for the poorly-known physical structure of the waists. We discuss some preliminary results from our data such as the nature of the illuminating sources, quantitative analysis of the mass and dynamics of different nebular components by combining our AO data with complementary data from our multi-wavelength survey of PPNs, and numerical simulations of precessing jets interacting with AGB winds.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.