The Bipolar Engines of post-AGB stars: Transient Dynamos and Common Envelopes

Abstract

The physical mechanism(s) responsible for transitioning from a spherical Asymptotic Giant Branch (AGB) star to an asymmetric post-AGB (pAGB) object is poorly understood. In particular, excess momenta in the outflows of pAGB objects suggest that a binary may be required to supply an additional source of energy and angular momentum. The extraction of rotational energy from the engine is likely fundamental and may be facilitated if a dynamo is operating in the interior. In this regard, single star magnetic outflow models have been proposed as mechanisms for producing and shaping PNe, however these models neglect the back-reaction of the large-scale magnetic field on the flow. Here we present a transient $α-Ω$ dynamo operating in the envelope of an AGB star in (1) an isolated setting and (2) a common envelope in which the secondary is a low-mass companion in-spiraling in the AGB interior. The back reaction of the fields on the shear is included and differential rotation and rotation deplete via turbulent dissipation and Poynting flux. For an isolated star, the shear must be resupplied in order to sufficiently sustain the dynamo. We comment on the energy requirements that convection must satisfy to accomplish this. For the common envelope case, a robust dynamo can result as the companion provides an additional source of energy and angular momentum.