Dynamics of magnetic flux tubes in evolved stars
Abstract
We explore the limits of the `solar paradigm' by applying the magnetic flux tube model, which proved to be consistent with many observed properties of sunspot groups, to subgiant and giant stars. The model starts from a strong toroidal field generated by a dynamo working at the bottom of the stellar convection zone. The field is stored within the stably stratified convective overshoot layer in the form of magnetic flux tubes. Once a critical field strength is exceeded, the undulatory (Parker-type) instability leads to flux loops rising through the convection zone. Upon emergence at the stellar surface, the flux loops form bipolar magnetic regions and starspots. We apply this model to evolved stars using evolutionary sequences for stars between 1 M_odot and 2.5 M_odot. We determine the stability limit by linear analysis und follow the development of the unstable flux loops through numerical simulation. We find that the Coriolis force leads to flux emergence at high stellar latitudes for stars with deep convection zones and large rotation rate. On the other hand, if the relative size of the stellar core falls short of a certain limit, i.e., at some stage of the rapid expansion of a star during the giant phase, flux loops formed by the instability do not surface any longer but find a new equilibrium within the star. This transition could possibly be connected with the existence of a `coronal dividing line' in the Hertzsprung-Russell diagram.
- Publication:
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Astronomische Gesellschaft Abstract Series
- Pub Date:
- 1999
- Bibcode:
- 1999AGAb...15R..71H
- Keywords:
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- Magnetic Flux Tubes: Dynamics