Dynamo Action and Magnetic Cycles in F-type Stars

Abstract : Magnetic activity and differential rotation are commonly observed features on main-sequence F-type stars. We seek to make contact with such observations and to provide a self-consistent picture of how differential rotation and magnetic fields arise in the interiors of these stars. The three-dimensional magnetohydrodynamic anelastic spherical harmonic code is employed to simulate global-scale convection and dynamo processes in a 1.2 M ☉ F-type star at two rotation rates. The simulations are carried out in spherical shells that encompass most of the convection zone and a portion of the stably stratified radiative zone below it, allowing us to explore the effects a stable zone has upon the morphology of the global-scale magnetic fields. We find that dynamo action with a high degree of time variation occurs in the star rotating more rapidly at 20 Ω☉, with the polarity of the mean field reversing on a timescale of about 1600 days. Between reversals, the magnetic energy rises and falls with a fairly regular period, with three magnetic energy cycles required to complete a reversal. The magnetic energy cycles and polarity reversals arise due to a linking of the polar-slip instability in the stable region and dynamo action present in the convection zone. For the more slowly rotating case (10 Ω☉), persistent wreaths of magnetism are established and maintained by dynamo action. Compared to their hydrodynamic progenitors, the dynamo states here involve a marked reduction in the exhibited latitudinal differential rotation, which also vary during the course of a cycle
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Submitted on : Tuesday, May 20, 2014 - 3:48:30 PM
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K.C. Augustson, A.S. Brun, J. Toomre. Dynamo Action and Magnetic Cycles in F-type Stars. The Astrophysical Journal, American Astronomical Society, 2013, 777 (2), pp.153. ⟨10.1088/0004-637X/777/2/153⟩. ⟨cea-00993726⟩

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