出版社：SISSA, Scuola Internazionale Superiore di Studi Avanzati
摘要：We present Very Long Baseline Array (VLBA) observations of the linear and circular polarization
of the H2O masers in the collimated jet of W43A. The observations reveal a strong toroidal
magnetic field, indicating that the jet is magnetically collimated. These are the first direct measurements
of the magnetic field strength and direction in a collimated jet from an evolved star on
its way to become a planetary nebula. The magnetic field strength in the jet extrapolated back
to the stellar surface is fully consistent with the magnetic fields determined from SiO, H2O and
OH maser polarization in a large sample of evolved stars. The origin of the magnetic field is
yet unknown, although the jet precession might point to the existence of a heavy planet or stellar
companion. Our observations thus provide strong evidence for magnetic collimation in the jets,
that likely play an important role in shaping planetary nebulae.
