Disk accretion onto a magnetized young star and associated jet formation

We investigated disk accretion onto a magnetized young star by performing MHD numerical simulations. We considered the case in which the stellar magnetosphere truncates the accretion disk carrying the interstellar magnetic field, and the star is magnetized in the same direction as the disk. In such a case, the interface between the accretion disk and the magnetosphere contains a magnetically neutral ring in the equatorial plane. The numerical results show that the disk accretion drives the magnetic reconnection between the magnetospheric field and the disk magnetic field, which allows mass transfer from the disk to the magnetosphere. Most of the transferred mass accretes to the star along the reconnected magnetospheric field, while the rest of the mass is accelerated to the bipolar directions by a Lorentz force along the stellar open magnetic field. This ''reconnection-driven'' jet is further accelerated magneto-centrifugally due to stellar rotation, and corresponds to optical jets from young stars. The magnetic braking as a reaction of the magneto-centrifugal acceleration of the jet may explain the observed slow rotations of young stars in the disk-accretion stage.