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R. Sahai (Caltech/ JPL), M. Morris (UCLA), C. Sanchez Contreras (CalTech), M. Claussen (NRAO)
The imaging of young planetary and pre-planetary nebulae (PNe and PPNe) with unprecedented high angular resolution and dynamic range using the Hubble Space Telescope (HST), has shown that almost all of these objects are highly aspherical, with complex multipolar morphologies. These data have forced radical changes in our understanding of the mass-loss processes during late stellar evolution, leading to a model for PN formation in which 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. Episodic changes in their orientation (or the operation of multiple outflows which operate quasi-simultaneously with different orientations) must be common in order to generate multipolar structures and/or point-symmetric morphologies fairly commonly in these objects.
In this paper, we describe the results from new large imaging surveys of young pre-planetary nebulae with HST. These surveys are part of a larger program of multiwavelength imaging and spectroscopic observations of the youngest PPNe from a large list (~300) of candidate objects. We have constructed a morphologically unbiased sample of such objects from catalogs of OH/IR stars, which are evolved, visually faint, mass-losing stars with dense circumstellar envelopes, showing (generally double-peaked) OH maser emission. The IRAS spectral energy distributions (SEDs) of a large fraction of these objects indicate a lack of hot dust (12 to 25\micron~flux ratio F12/F25<1), implying a recent decrease in their mass-loss rates, marking the beginning of their post-AGB evolution. We present the results to date of these imaging studies -- these confirm our expectation that the seeds of asymmetry are sown very early in the transition of AGB stars to pre-planetary nebulae.
The author(s) of this abstract have provided an email address for comments about the abstract: raghvendra.sahai@jpl.nasa.gov
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.