Abstract
Using recent results on the operation of turbulent dynamos, we show that a turbulent dynamo may 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 αω dynamos, the rotation has only a small role in amplifying the toroidal component of the magnetic field; instead of an αω dynamo we propose an α2ω. The magnetic field may reach a value of B ≃ 10-4Be ≃ 0.01 G, where Be is the equipartition (between the turbulent and magnetic energy densities) magnetic field. The large-scale magnetic field is strong enough for the formation of magnetic cool spots on the AGB stellar surface. The spots may regulate dust formation, and hence the 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, as required by some models. We discuss other possible 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 non-spherical PNe.
Original language | English |
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Pages (from-to) | 204-208 |
Number of pages | 5 |
Journal | Monthly Notices of the Royal Astronomical Society |
Volume | 329 |
Issue number | 1 |
DOIs | |
State | Published - 1 Jan 2002 |
Externally published | Yes |
Keywords
- Circumstellar matter
- Planetary nebulae: general
- Stars: AGB and post-AGB
- Stars: magnetic fields
- Stars: mass-loss