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Paper: The Geometrical, and Magnetic Structure of the Proto-Planetary Nebula OH 231.8+4.2 Traced by OH Maser Emission
Volume: 404, The Eighth Pacific Rim Conference on Stellar Astrophysics: A Tribute to Kam Ching Leung
Page: 311
Authors: Etoka, S.; Zijlstra, A.; Richards, A.M.; Matsuura, M.; Lagadec E.
Abstract: Evolved stars during the AGB and especially the post-AGB phase develop markedly non-spherical circumstellar shells, evolving later into planetary nebulae. The physical mechanisms causing this runaway change in morphology from spherical to aspherical are still disputed: binarity, rotation and magnetic fields are being considered. OH 231.8+4.2 is the prototype of the bipolar protoplanetary nebulae. Observations over a wide range of wavelengths have revealed its structure over size scales ranging from milliarcseconds to arcminutes. Masers are a particularly powerful probe to explore both the geometrical structure of the circumstellar material and the dynamics, as they yield high angular and velocity resolution. For instance, SiO masers revealed the presence of a significant rotation near the central object, as well as infalling material. OH masers found far away from the star can also probe the polarimetric properties at the maser location. We present new MERLIN observations in the 1667-MHz OHmaser line. From this we determine the velocity field of the outflowing material, and the polarimetric structure. This revealed a well-organized magnetic field embedded in the outflow, possibly originating from the compact core suggested by recent infrared observations. The correlation found between the geometric structure and the magnetic field orientation is indicative of the significant role of the magnetic field in the shaping process.
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