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 dis c 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 anal ysis is made of the outflows and their back effect on the disc structu re, assuming an 'a' turbulent viscosity model for the disc and a magne tic diffusivity comparable to this viscosity. The outflows are found t o extend over a range of radial distances inward to a distance close t o 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 mo ment, the accretion rate, and the disc's magnetic diffusivity. The out flow regime is accompanied in general by a spin-up of the rotation rat e 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, b ut instead that 'magnetic braking' of the star by the disc due to fiel d-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 sta r, depending mainly on the star's magnetic moment and the mass accreti on rate. For a system with r(to) comparable to r(cr), bimodal behaviou r is possible where extraneous perturbations (for example, intermitten cy of alpha, B field flux introduced from the companion star, or varia tions in the mass accretion rate) cause the system to flip between spi n-up (with outflows, r(to) < r(cr)) and spin-down (or spin-up) (with n o outflows, r(to) > r(cr)).