Bipolar planetary nebulae: The Betes-Noires of hydro models

Interacting Stellar Winds ("ISW") scenarios generally invoke a dense ring or torus of gas near the central star in order to collimate a (presumed) isotropic wind outflow that can account for the shapes of bipolar PNe, bPNe. However, HST/WFPC2 images of bPNe show embarrassing gaps between the nucleus and the inner portions of the collimated flow in more than half of known bipolars. In other cases a possible nozzle is resolved on scales of 10(15) cm or larger. However, this size scale is much larger than expected for scenarios based on tidal forces and spin up by an orbiting companion. Magnetic collimation also seems consistent with at least some of the images of bipolar PNe. But models tests need to be devised and executed before the concept is credible.