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Paper: Optical Imaging of Proto-planetary Nebulae
Volume: 199, Asymmetrical Planetary Nebulae II: From Origins to Microstructures
Page: 155
Authors: Hrivnak, Bruce J.; Kwok, Sun; Su, Kate Y. L.
Abstract: Evidence has been accumulating over the past decade which indicates that most PPNs have aspherical shapes. HST imaging shows this clearly, revealing that many possess a bipolar symmetry. Examples are shown of bipolar PPNs viewed edge-on and at intermediate orientations. Several are surrounded by concentric arcs or rings. A large ratio of the mid-IR to visible brightness in the spectral energy distribution is found to be a good indicator of a bipolar nebula viewed at a large inclination.
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