Toroidal magnetic fields around planetary nebulae.

Many planetary nebulae have asymmetric winds, often forming bipolar outflow lobes or jets. One theory for this phenomenon is that differential rotation of the envelope twists the star's magnetic field into a toroidal flux tube, which channels the fast winds. Direct testing of this theory is now possible using submillimetre polarimetry of magnetically aligned dust grains, and such a test has been made for the planetary nebula NGC 7027 and the proto-planetary nebula CRL 2688. The results show that the magnetic field is within 15 degrees of toroidal towards NGC 7027, but about 35 degrees off-axis for CRL 2688. However, the telescope beam size of 1500 is well matched to the size of the jet base in NGC 7027 and poorly so in CRL 2688 where the observation is more sensitive to the extended envelope. Magnetic field directions in the envelopes are not well aligned with the outflow axes. The polarization percentage is an order of magnitude less towards NGC 7027 than in the outer envelope, so assuming similar grain alignment efficiencies everywhere, the scale of the organised toroidal field is of the order of 5000 AU.