Thermo-centrifugal wind from a rotating magnetic dipole

We present numerical solutions for axisymmetric thermo-centrifugal winds with a dipole magnetic flux distribution on the stellar surface. Parameters of the models are chosen to be those typical for the Sun, except for the angular velocity Ω0, which is increased to up to 60 times the solar value. The dipole moment is not increased with Ω0, but is kept constant. The value of β of the plasma is about unity on the stellar surface, and the range of the ratio Ω0 ωA/Cs is 0–2.6, where ωA is the equatorial radius of the Alfvén surface and Cs is the sound velocity. As the value of Ω0 is increased to more than about 10 times the solar value, the centrifugal (electromagnetic) driving force becomes comparable with the thermal driving force, and the meridional structure of the wind is then strongly influenced by the electromagnetic force (the thermo–centrifugal wind). The magnetic pressure of the toroidal field produced by the stellar rotation is maximized at medium latitudes because our source field is dipolar. As a result, the following meridional structure emerges: (i) the flow is not only deflected towards the rotational axis but is also pressed towards the equatorial plane; (ii) centrifugal acceleration takes place most efficiently at medium latitudes; and (iii) the mass loss is enhanced in both the polar and equatorial regions. These results are different from previous results obtained using the split-monopole model, in which the mass loss is maximal on the rotation axis only, and the acceleration is most efficient at the equator. The loss rates of energy, angular momentum and mass are obtained as functions of Ω0. We find, for the angular momentum loss rate, |J=4.5× 10²⁸(Ω₀/Ωₛ)1.5B₀0.9| erg, for Ω0 = (20–60 )Ωs, where Ωs is the solar angular velocity and B0 is the polar magnetic field strength of between 0.5 and 2 G.