Magnetic reconnection in a magnetosphere-accretion-disk system. Axisymmetric stationary states and two-dimensional reconnection simulations.

In the present paper we investigate the transport of accreting plasma across the magnetopause onto a strongly magnetized massive star (i.e. white dwarf or neutron star) by magnetic reconnection. A simplified axisymmetric magnetic field model of an aligned rotator is used to study the reconnection process. To be able to separate effects caused by instabilities of the system from intrinsic time-dependent behaviour, we first construct self-consistent stationary states of the magnetosphere-disk system. We include a rigid magnetospheric rotation and Keplerian rotation of the magnetized disk plasma.

The stationary states are computed numerically with a relaxation method which conserves the magnetic topology. Therefore we can prescribe an initial condition of the relaxation process using a magnetic field consisting of a dipole of the compact object and a homogeneous field threading the disk. The magnetopause then separates the regions of closed field lines with corotating plasma from open field lines with plasma in Keplerian motion.

The resistive stability of the stationary states is examined by two-dimensional magnetohydrodynamic simulations. We find that magnetic reconnection leads to mass transport across the magnetopause onto closed magnetic field lines The accretion disk material is accelerated along the magnetic field lines that are connected to the magnetic poles of the compact object and will eventually be accreted by the star at its polar caps.