MERLIN polarimetry of the OH masers in OH17.7-2.0
We report the first results of our study of magnetic fields in protoplanetary nebula candidates. Such fields may have important implications for the currently accepted Generalized Interacting Stellar Winds path to planetary nebula formation. We present the first images of the 1612 and 1667 MHz full polarization OH maser emission from the protoplanetary nebula, OH17.7–2.0, which were observed with MERLIN, at an angular resolution of 0.2 arcsec. We have detected one fully-resolved Zeeman σ-component pair at each observing frequency, from which we measure magnetic field strengths of B=+4.6 mG (1612 MHz) and B=+2.5 mG (1667 MHz) . This is the first time a magnetic field strength has been measured in a protoplanetary nebula. The observations suggest a large-scale, regular magnetic field structure consistent with that of a stretched dipole field. When considered with near-infrared dust imaging polarimetry data, OH17.7–2.0 is shown to be a prolate spheroid, inclined at 20° to the sky plane, with the polar (major axis) outflow at a position angle of 20° in the plane of the sky and the denser equatorial axis at a position angle of between 120° in the same plane. The masers are shown to be contained in, and aligned with, this equatorial axis. There is additional 1612 MHz OH maser emission that can be associated with the interaction region between the fast wind from the post-AGB star and the remnant AGB shell. These data show that the stellar magnetic field can produce the necessary latitude-dependent mass loss on the AGB that is required in addition to the Generalized Interacting Stellar Winds models to produce ellipsoidal and bipolar planetary nebulae.