Spin-down of protostars through gravitational torques

Young protostars embedded in circumstellar discs accrete from an angular momentum rich mass reservoir. Without some braking mechanism, all stars should be spinning at or near breakup velocity. In this paper, we perform simulations of the self-gravitational collapse of an isothermal cloud using the orion adaptive-mesh refinement code and investigate the role that gravitational torques might play in the spin-down of the dense central object. While magnetic effects likely dominate for low-mass stars, high-mass and Population III stars might be less well magnetized. We find that gravitational torques alone prevent the central object from spinning up to more than half of its breakup velocity, because higher rotation rates lead to bar-like deformations that enable efficient angular momentum transfer to the surrounding medium. We also find that the long-term spin evolution of the central object is dictated by the properties of the surrounding disc. In particular, spiral modes with the azimuthal wavenumber m= 2 couple more effectively to its spin than the lopsided m= 1 mode, which was found to inhibit spin evolution. We suggest that even in the absence of magnetic fields, gravitational torques may provide an upper limit on stellar spin, and that moderately massive circumstellar discs can cause long-term spin-down.