Toroidal Stages in Stellar Evolution

Abstract

ONE of the questions which the modern theory of stellar evolution fails to explain is why there are no stars rotating with extreme equatorial velocities. It is easy to see that the rotation of protostars is restricted because of the conservation of angular momentum during contraction, but if the type of rotation remains unchanged, angular velocity should be connected with the radius of a protostar by the relationship where the subscript refers to initial values. The limiting angular velocity ω_lim is given by Taking we obtain ω≈ω_lim if R ∼ 0.4R₀, that is, the rotation of the protostar is restricted before the star itself is formed. Thus the angular momentum of a protostar exceeds that of main sequence stars by 10⁴. Removing this difficulty by taking magnetic fields into consideration, we are faced with another problem : why magnetic stars rotate rapidly when they ought to have lost their angular momentum. Neither is it easy to explain why the rotational velocities of stars of early spectral type increase with the radius of a star. Problems in the origin of close binary systems are also well known. We would like to point out a further difficulty in the polar character of the stellar magnetic field.