Planetary nebulae often have asymmetric shapes, even though their progenitor stars were symmetric; this structure could be the result of collimated jets from the evolved stars before they enter the planetary nebula phase1,2,3. Theoretical models have shown that magnetic fields could be the dominant source of jet-collimation in evolved stars4,5, just as these fields are thought to collimate outflows in other astrophysical sources, such as active galactic nuclei6,7,8,9 and proto-stars10,11. But hitherto there have been no direct observations of both the magnetic field direction and strength in any collimated jet. Here we report measurements of the polarization of water vapour masers that trace the precessing jet emanating from the asymptotic giant branch star W43A (at a distance of 2.6 kpc from the Sun), which is undergoing rapid evolution into a planetary nebula2,12. The masers occur in two clusters at opposing tips of the jets, ∼1,000 au from the star. We conclude from the data that the magnetic field is indeed collimating the jet.
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NRAO is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc. W.H.T.V. was supported by a Marie-Curie Intra-European fellowship.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
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