MRI-driven Accretion on to Magnetized stars: Global 3D MHD Simulations of Magnetospheric and Boundary Layer Regimes

Abstract

We discuss results of global 3D MHD simulations of accretion on to a rotating magnetized star with a tilted dipole magnetic field, where the accretion is driven by the magneto-rotational instability (MRI). The simulations show that MRI-driven turbulence develops in the disc, and angular momentum is transported outwards due primarily to the magnetic stress. The turbulent flow is strongly inhomogeneous and the densest matter is in azimuthally-stretched turbulent cells. We investigate two regimes of accretion: a magnetospheric regime and a boundary layer (BL) regime. In the magnetospheric regime, the accretion disc is truncated by the star's magnetic field within a few stellar radii from the star, and matter flows to the star in funnel streams. The funnel streams flowing towards the south and north magnetic poles but are not equal due to the inhomogeneity of the flow. In the BL regime, matter accretes to the surface of the star through the boundary layer. The magnetic field in the inner disc is strongly amplified by the shear of the accretion flow, and the matter and magnetic stresses become comparable. Accreting matter forms a belt-shaped region on the surface of the star. The belt has inhomogeneous density distribution which varies in time due to variable accretion rate. Results of simulations can be applied to classical T Tauri stars, accreting brown dwarfs, millisecond pulsars, dwarf novae cataclysmic variables, and other stars with magnetospheres smaller than several stellar radii.