The central problem in forming a star is the angular momentum in the circumstellar disk, which prevents material from falling into the central stellar core. An attractive solution to the angular momentum problem appears to be the ubiquitous (low-velocity and poorly collimated) molecular outflows and (high-velocity and highly collimated) protostellar jets accompanying the earliest phase of star formation that remove angular momentum at a range of disk radii1. Previous observations have suggested that outflowing material carries away the excess angular momentum via magneto-centrifugally driven winds from the surfaces of circumstellar disks down to ˜10 au scales2,3,4,5,6, allowing the material in the outer disk to be transported to the inner disk. Here we show that highly collimated protostellar jets remove the residual angular momenta at the ˜0.05 au scale, enabling the material in the innermost region of the disk to accrete towards the central protostar. This is supported by the rotation of the jet measured down to ˜10 au from the protostar in the HH 212 protostellar system. The measurement implies a jet launching radius of ˜0.05-0.02 0.05 au on the disk, based on the magneto-centrifugal theory of jet production, which connects the properties of the jet measured at large distances with those at its base through energy and angular momentum conservation7.
