Spin-down of protostars through gravitational torques
We present three dimensional hydrodynamic simulations of star-disc systems, focusing on the angular momentum evolution of the central object due to gravitational interactions with the disc. It is found that stellar spin-up is self-limited to approximately half its break-up speed. On long time-scales, we find that in simulations where m=1 is the dominant non-axisymetric mode, there is limited evolution in stellar spin. By contrast, in simulations where m=1 is non-dominant, we observe a monotonic decrease in stellar spin. Our experiments suggest a necessary condition for long-term spin down be that the system does not develop significant m=1 mode, which displaces the star from its center of mass.