Extremely energetic planetary nebulae in the Large Magellanic Cloud.

Highly energetic planetary nebulae have been identified in the Large Magellanic Clouds. These have higher expansion velocities than any of their galactic counterparts and appear to show directed expansion flows rather than having a spherically symmetric expanding shell. Comparison of their properties with similar, but less extreme, galactic planetaries suggests that these objects are highly luminous bipolar type I planetaries in which the central star is undergoing continued energetic mass loss, but which is nevertheless insufficient to supply the energy and momentum of the nebula. This class of objects is expected to represent the upper end of the mass range of precursor stars which are able to evolve to the planetary nebula phase, and circumstantial evidence connecting them with the symbiotic stars is presented.