MHD simulations of accretion onto a dipolar magnetosphereI. Accretion curtains and the disk-locking paradigm

摘要

Aims. We investigate the accretion process from an accretion disk onto a magnetized rotating star with a purely dipolar magnetic field.Our main aim is to study the mechanisms that regulate the stellar angular momentum. In this work, we consider two effects that cancontrast with the spin-up torque normally associated with accretion: (1) the spin-down torque exerted by an extended magnetosphereconnected to the disk beyond the corotation radius; (2) the spin-down torque determined by a stellar wind flowing along the openedmagnetospheric field lines.Methods. Our study is based on time-dependent axisymmetric magnetohydrodynamic numerical simulations of the interaction be-tween a viscous and resistive accretion disk with the dipolar magnetosphere of a rotating star. We present the first example of anumerical experiment able to model at the same time the formation of accretion curtains, the effects of an extended stellar magneto-sphere and the launching of a stellar wind.Results. In the examples presented, the spin-down torque related to the star-disk interaction can extract only -10 of the accretiontorque, due to the weakness of the extended connection. Not even the spin-down torque exerted by a stellar wind is strong enough(~20): despite a huge lever arm (R_A≈19 R_*), the mass-loss rate(Mwind ≈1 Macc)is too low to provide an efficient torque.MaM⊙yr~(-1),B_*,dipoleConclusions. We argue that, at least in the case of typical classical T Tauri stars (1 kG) rotatingat 10 of their break-up speed, the disk spin-down is unlikely to balance the accretion torque ("disk locked" equilibrium). A mas-sive stellar wind (M_(wind)≈20M_(acc))could in principle succeed, but its mass and energy fluxes are quite demanding, both from atheoretical and an observational point of view.