MRI-driven accretion on to magnetized stars: axisymmetric MHD simulations

We present the first results of a global axisymmetric simulation of accretion on to rotating magnetized stars from a turbulent accretion disc, where the turbulence is driven by the magnetorotational instability (MRI). We observed that the angular momentum is transported outwards by the magnetic stress and accretion rate corresponds to a Shakura-Sunyaev viscosity parameter alpha approximate to 0.01-0.04. The disc is stopped by the magnetic pressure of the magnetosphere, and matter flows on to the star in funnel streams, which usually choose a path along top or bottom side of the magnetosphere. The character of accretion depends on a number of factors, including the size of the magnetosphere, the accretion rate and orientation of the initial disc poloidal field relative to the star's field at the disc-magnetosphere boundary. If fields have same direction (are parallel), then the magnetic flux is accumulated at the boundary and blocks accretion which leads to the accumulation of matter at the boundary. Subsequently, this matter accretes to the star in bursts before accumulating again. Hence, the cyclic, 'bursty' accretion is observed. The magnetic stress is enhanced at the boundary, leading to the enhanced accretion rate. In the opposite case of antiparallel fields, the fields of the disc and the star reconnect at the boundary, the magnetic stress in the inner disc is lower, and matter accretes smoothly and at a lower rate. Test simulations show that in the case of a higher accretion rate corresponding to alpha = 0.05-0.1, accretion is bursty in cases of both field directions. We conclude that the episodic, bursty accretion is expected in stars with larger magnetospheres, higher accretion rates and parallel fields. In stars with small magnetospheres, accretion is usually smoother. However, when the disc comes close to the surface of the star, we observe a different type of bursty accretion, where periods of boundary layer accretion alternate with periods of magnetospheric expansion. In the case of even smaller magnetospheres, matter accretes smoothly through the boundary layer. Accretion may alternate between bursty and smooth, if the disc accretion rate varies with time, and/or if it brings the poloidal field of alternating polarity. We find that a rotating, magnetically dominated corona forms above and below the disc, and that it slowly expands outwards, driven by the magnetic force.