Does Disk Locking Solve the Stellar Angular Momentum Problem?

We critically examine the theory of disk locking, which assumes that the angular momentum deposited on an accreting protostar is exactly removed by torques carried along magnetic field lines connecting the star to the disk. In this letter, we consider that the differential rotation between the star and disk naturally leads to an opening (i.e., disconnecting) of the magnetic field between the two. We find that this significantly reduces the spin-down torque on the star by the disk. Thus, disk-locking cannot account for the slow rotation ($\sim$10% of breakup speed) observed in several systems and for which the model was originally developed.