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M.- M. Mac Low (AMNH), G. Garc\'{i}a-Segura (UNAM-Ensenada)
Bipolar nebulae occur around many post-main sequence stars, both low-mass (bipolar planetary nebulae) and massive (LBV nebulae such as \eta Carinae). We here review how the combined effects of rotation and magnetic fields, along with photoionization, can explain the myriad of observed shapes in a natural and self-consistent way. Rotation acts by forming equatorial density enhancements in winds from stars rotating near the critical velocity for liftoff of material from the equator. Subsequent faster winds expanding into the resulting axisymmetric density distributions will form bipolar bubbles. Magnetic fields act when toroidal fields in the wind contract to collimate the flow toward the axis, producing the ansae and short jets observed in some planetary nebulae. Neither mechanism acts with the vigor observed in pre-main sequence stars, presumably due to the lack of massive accretion disks in post-main sequence stars.
The author(s) of this abstract have provided an email address for comments about the abstract: mordecai@amnh.org