SPIN-UP SPIN-DOWN OF MAGNETIZED STARS WITH ACCRETION DISCS AND OUTFLOWS

An investigation is made of disc accretion of matter on to a rotating star with an aligned dipole magnetic field. A new aspect of this work is that we argue that, when the angular velocities of the star and disc differ substantially, the B field linking the star and disc rapidly inflates to give regions of-open field lines extending from the polar caps of the star and from the disc. The open field line region of the disc leads to the possibility of magnetically driven outflows. An analysis is made of the outflows and their back effect on the disc structure, assuming an 'a' turbulent viscosity model for the disc and a magnetic diffusivity comparable to this viscosity. The outflows are found to extend over a range of radial distances inward to a distance close to r(to), which is the distance of the maximum of the angular rotation rate of the disc. We find that r(to) depends on the star's magnetic moment, the accretion rate, and the disc's magnetic diffusivity. The outflow regime is accompanied in general by a spin-up of the rotation rate of the star. When r(to) exceeds the star's corotation radius r(cr) =(GM/omega(*)(2))(1/3), we argue that outflow solutions do not occur, but instead that 'magnetic braking' of the star by the disc due to field-line twisting occurs in the vicinity of r(cr). The magnetic braking solutions can give spin-up or spin-down (or no spin change) of the star, depending mainly on the star's magnetic moment and the mass accretion rate. For a system with r(to) comparable to r(cr), bimodal behaviour is possible where extraneous perturbations (for example, intermittency of alpha, B field flux introduced from the companion star, or variations in the mass accretion rate) cause the system to flip between spin-up (with outflows, r(to) < r(cr)) and spin-down (or spin-up) (with no outflows, r(to) > r(cr)).