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

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 can contrast with the spin-up torque normally associated with accretion: (1) the spin-down torque exerted by an extended magnetosphere connected to the disk beyond the corotation radius; (2) the spin-down torque determined by a stellar wind flowing along the opened magnetospheric field lines. Methods. Our study is based on time-dependent axisymmetric magnetohydrodynamic numerical simulations of the interaction between a viscous and resistive accretion disk with the dipolar magnetosphere of a rotating star. We present the first example of a numerical experiment able to model at the same time the formation of accretion curtains, the effects of an extended stellar magnetosphere 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 similar to 10% of the accretion torque, due to the weakness of the extended connection. Not even the spin-down torque exerted by a stellar wind is strong enough (similar to 20%): despite a huge lever arm (R(A) approximate to 19 R(star)), the mass-loss rate ((M) over dot(wind) approximate to 1% (M) over dot(acc)) is too low to provide an efficient torque. Conclusions. We argue that, at least in the case of typical classical T Tauri stars ((M) over dot(acc) approximate to 10(-8) M(circle dot) yr(-1), B(star),(dipole) less than or similar to 1 kG) rotating at 10% of their break-up speed, the disk spin-down is unlikely to balance the accretion torque (disk locked equilibrium). A massive stellar wind ((M) over dot(wind) approximate to 20%. (M) over dot(acc)) could in principle succeed, but its mass and energy fluxes are quite demanding, both from a theoretical and an observational point of view.