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Evolution of Magnetic Flux Tubes in Convective Envelopes of Close Binary Stars

Samujłło M., Kiraga M.

Acta Astronomica

2007

Vol. 57, No. 4

347--367

Observations of cool stars with very high activity levels show a tendency of magnetic regions to be concentrated near the poles. In close binaries a preference of low latitude regions to appear at fixed longitudes has also been suggested. We modeled a 3D process of emergence of a thin magnetic tube through a convective envelope of a solar type component of a close binary. In no case the emerging tube approached a pole by more than 20°, compared to its initial latitude. Assuming that dynamo operates mainly at low and moderate latitudes, the polar magnetic regions cannot be formed unless an additional poleward transport of magnetic fields exists in rapidly rotating stars. Preferred longitudes in active close binaries result in our model from an azimuthal variation of effective gravity, hence buoyancy, throughout the convection zone of each component. The variation is strong enough in a contact binary with orbital period of 0.3 days to force parts of the tube emerging in the regions of enhanced buoyancy to approach stellar surface substantially sooner than the rest of the tube, thus forming two preferred longitudes 180° apart. The precise positions of the preferred longitudes depend on binary parameters. The gravity variation is, however, too weak in a close binary with a period of 2 days to influence significantly the emergence rate of different tube parts. A magnetic region can form at any longitude, depending mostly on an initial perturbation, provided the original tube is placed on an equipotential surface. Such a location of the tube seems to be a correct initial condition. The tube originally placed on a spherical surface has parts lying closer to the unstable zone, where the tube instability preferentially develops. This results in magnetic flux emergence at preferred longitudes.

Dynamical Evolution of Magnetic Flux Tube in the Solar Convective Zone

Samujłło M.

Acta Astronomica

1998

Vol. 48, No. 4

789--802

We present computations of nonlinear development of the kink instability in a horizontal magnetic flux tube lying initially in the deep convective zone. Obtained results are compared with an independent solution of the problem obtained formerly by Moreno-Insertis (1986). Consistency of solutions allows to evaluate the numerical code, in particular the treatment of the energy balance which is different than that of Moreno-Insertis.