Why Magnetic Fields Cannot Be the Main Agent Shaping Planetary Nebulae

An increasing amount of literature reports the detection of magnetic fieldsin asymptotic giant branch (AGB) stars and in central stars of planetarynebulae (PNs). These detections lead to claims that the magnetic fields are themain agent shaping the PNs. In this paper, I examine the energy and angularmomentum carried by magnetic fields expelled from AGB stars, as well as otherphysical phenomena that accompany the presence of large scale fields, such asthose claimed in the literature. I show that a single star cannot supply theenergy and angular momentum if the magnetic fields have the large coherentstructure required to shape the circumstellar wind. Therefore, the structure ofnon-spherical planetary nebulae cannot be attributed to dynamically importantlarge scale magnetic fields. I conclude that the observed magnetic fieldsaround evolved stars can be understood by locally enhanced magnetic loops whichcan have a secondary role in the shaping of the PN. The primary role, I argue,rests with the presence of a companion.Comment: PASP, 2006, in press. (This paper was rejected by MNRAS and ApJ; my criticism of the referee reports are in: Soker, N. astro-ph/0508525.