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J.H. Kastner (Chester F. Carlson Center for Imaging Science, Rochester Institute of Technology)
Planetary nebulae (PNs) and protoplanetary nebulae (PPNs) represent the final stages of high-luminosity evolution in the life cycles of intermediate-mass (~1-8 M\odot) stars. Observations of PNs and PPNs over a wide range of wavelengths, from the radio to the X-ray, afford the opportunity to witness the dispersal of stardust (and gas) into the interstellar medium (ISM). Among many other recent developments in the study of such objects, it is increasingly apparent that the mass ejection process can change from spherically symmetric (if episodic) to highly anisotropic and energetic, during late asymptotic giant branch (AGB), PPN, and early PN evolutionary stages. Such transformations may have profound impact on the rate of mass return to the ISM, as well as on the chemical constituents of this recycled material.
Results from recent observations of the well-studied young PN NGC 7027 well illustrate the emergence of collimated outflows. Deep optical, infrared, and radio interferometric images of NGC 7027 reveal a symmetrical system of dusty, molecule-rich shells surrounding the elliptical, ionized nebula. The mass that constitutes these structures presumably was ejected while the progenitor star was on the AGB, and these ejecta are now being photodissociated and ionized by this same star as its hot core is unveiled. Meanwhile, fast, collimated flows protrude from the central, elliptical PN, disturbing the symmetry of the inner nebula and generating X-rays as they shock the slower-moving nebular gas. Such a complex superposition of structures, which is also observed in many other objects, points to the potential importance of close companions and/or stellar magnetic fields as the primary agents of the rapid structural transformations of PPNs and PNs.
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Bulletin of the American Astronomical Society, 37 #2
© 2005. The American Astronomical Soceity.