Rotation Law and Magnetic Field for M Dwarf Models

Summary

In stellar convection zones and fully convective stars, the rotation profiles are determined by the balance between the Reynolds stress and the meridional circulation. Due to the Coriolis force, the Reynolds stress has a non-diffusive component called ∧-effect that drives both differential rotation and meridional motions. The solar differential rotation pattern is almost perfectly reproduced by a mixing-length model of the convection zone that takes into account the influence of the Coriolis force on the convective motions. The same model also yields the turbulent electromotive force that together with rotational shear drives the solar dynamo.

The model has recently been applied to a fully convective pre-main sequence star. We find that for a strictly spherical star without any latitudinal gradients in temperature, density and pressure the rotation is very close to the rigid-body state. We conclude that the stellar magnetic field must be generated by a mechanism quite different from that in the Sun, namely an α² rather than an αΩ-dynamo. It is thus very likely to have non-axisymmetric geometry and not to show cyclic behavior.

We study the analogous problem for M dwarfs. Like the T Tauri stars, these objects are fully convective and may hence be expected to have similar rotational profiles and magnetic field structures, respectively. As their Coriolis numbers are, however, closer to solar values than to those of pre-main sequence stars, the rotation may also be of solar-type.