The action of magnetic torques on the oblique rotator model for magnetic Ap stars

The oblique magnetic rotator is a satisfactory model describing the general properties of the magnetic Ap stars. The inferred rotation periods are almost always considerably longer than those of the normal A stars. The loss of angular momentum is plausibly the result of magnetic coupling with external gas during some epoch in the star’s life. The same Maxwell stresses that brake the stellar rotation cause a precession of the instantaneous axis of rotation, so reducing the obliquity angle between the rotational and magnetic axes. A star that has suffered excess braking in the pre-main-sequence Hayashi phase may be magnetically spun up on the main sequence, with a simultaneous increase in the obliquity. This paper constructs the braking and precessional torques for an idealized illustrative model. The results may be relevant to recent observational inferences of a correlation between long rotation period and small obliquity.