3D MHD Simulations of Accretion onto Stars with Tilted Magnetic and Rotational Axes

Abstract: We present results of global three-dimensional (3D) magnetohydrodynamic (MHD) simulations of accretion onto magnetized stars where both the magnetic and rotational axes of the star are tilted about the rotational axis of the disc. We observed that initially the inner parts of the disc are warped, tilted, and process due to the magnetic interaction between the magnetosphere and the disc. Later, larger tilted discs form with the size increasing with the magnetic moment of the star. The normal vector to the discs are tilted at different angles, from 5-10 degrees up to 30-40 degrees. Small tilts may result from the winding of the magnetic field lines about the rotational axis of the star and the action of the magnetic force which tends to align the disc. Another possible explanation is the magnetic Bardeen-Petterson effect in which the disc settles in the equatorial plane of the star due to precessional and viscous torques in the disc. Tilted discs slowly precess with the time scale of the order of 50 Keplerian periods at the reference radius (approx. 3 stellar radii). Our results can be applied to different types of stars where signs of tilted discs and/or slow precession have been observed.