MHD simulations of accretion onto a dipolar magnetosphere

Abstract

Aims: This paper examines the outflows associated with theinteraction of a stellar magnetosphere with an accretion disk. Inparticular, we investigate the magnetospheric ejections (MEs) due to theexpansion and reconnection of the field lines connecting the star withthe disk. Our aim is to study the dynamical properties of the outflowsand evaluate their impact on the angular momentum evolution of youngprotostars. Methods: Our models are based on axisymmetrictime-dependent magnetohydrodynamic simulations of the interaction of thedipolar magnetosphere of a rotating protostar with a viscous andresistive disk, using alpha prescriptions for the transportcoefficients. Our simulations are designed to model the accretionprocess and the formation of accretion funnels, the periodicinflation/reconnection of the magnetosphere and the associated MEs, andthe stellar wind. Results: Similar to a magnetic slingshot, MEscan be powered by the rotation of both the disk and the star so thatthey can efficiently remove angular momentum from both. Depending on theaccretion rate, MEs can extract a relevant fraction of the accretiontorque and, together with a weak but non-negligible stellar wind torque,can balance the spin-up due to accretion. When the disk truncationapproaches the corotation radius, the system enters a ''propeller''regime, where the torques exerted by the disk and the MEs can evenbalance the spin-up due to the stellar contraction. Conclusions:Magnetospheric ejections can play an important role in the stellar spinevolution. Their spin-down efficiency can be compared to otherscenarios, such as the Ghosh {\amp} Lamb, X-wind, or stellar wind models.Nevertheless, for all scenarios, an efficient spin-down torque requires a rather strong dipolar component, which has seldom been observed in classical T Tauri stars. A better analysis of the torques acting on the protostar mustconsider non-axisymmetric and multipolar magnetic components consistentwith observations.