Turbulent Dynamo in Asymptotic Giant Branch Stars

Abstract

Using recent results on the operation of turbulent dynamos, we show that a turbulent dynamo can amplify a large scale magnetic field in the envelopes of asymptotic giant branch (AGB) stars. We propose that a slow rotation of the AGB envelope can fix the symmetry axis, leading to the formation of an axisymmetric magnetic field structure. Unlike solar-type alpha-omega dynamos, the rotation has only a small role in amplifying the toroidal component of the magnetic field. The large-scale magnetic field is strong enough for the formation of magnetic cool spots on the AGB stellar surface. The spots can regulate dust formation, hence mass loss rate, leading to axisymmetric mass loss and the formation of elliptical planetary nebulae (PNe). Despite its role in forming cool spots, the large scale magnetic field is too weak to play a dynamic role and directly influence the wind from the AGB star. We find other problems in models where the magnetic field plays a dynamic role in shaping the AGB winds, and argue that they cannot explain the formation of nonspherical PNe.