I am Rajika Kuruwita, an Independant Postdoc Fellow
at the Heidelberg Institute for Theoretical Studies

Abstract: Context. Many fast rotating stars (rotation periods of less than 2 days) are found to be unresolved binaries with separations of tens of au. This correlation between fast rotators and binarity leads to the question of whether the formation of binary stars inherently produces fast rotators. Aims. Our goal is to understand the spin evolution of protostars and whether the formation of companions plays a role in spinning up stars. Methods. We use magneto-hydrodynamical simulations to study the formation of multiple star systems from turbulent and non-turbulent protostellar cores. We track the angular momentum accreted by individual star and inner disc systems by using a sink (star) particle technique. We run a resolution study to extrapolate protostellar properties. Results. We find in all simulations that the primary star can experience a spin-up event correlated with the formation of companions, i.e., fragmentation into binaries or higher-order systems. The primary star can spin up by up to 84% of its pre-fragmentation angular momentum and by up to 18% of its pre-fragmentation mass-specific angular momentum. The mechanism for the spin-up is gravitational disc instabilities in the circumstellar disc around the primary star, leading to the accretion of material with high specific angular momentum. The simulations that experience the strongest disc instabilities fragment to form companions. Simulations with weaker spin-up events experience disc instabilities triggered by a companion flyby, and the disc instability in these cases typically does not produce further fragments, i.e., they remain binary systems. Conclusions. The primary star in multiple star systems may end up with a higher spin than single stars. This is because gravitational instabilities in the circumstellar disc around the primary star can trigger a spin-up event. In the strongest spin-up events, the instability is likely to cause disc fragmentation and the formation of companions. This spin-up mechanism, coupled with shorter disc lifetimes due to truncated circumstellar discs (and thus short spin-down times), may help produce fast rotators.

Both movies show the gas density (left), magnetic Toomre Q (middle), and spicific angular momenum (i.e.) spin of the primary star in these systems. Both movies show the evolution of these quantities as seen in the simulation with the strongest spin-up event (Top; 18% increase), and the weakest spin-up event (Bottom; 5%). We see in the Strong event, the circumstellar disc become gravitationally unstable triggering the spin-up event, and a companion forms as a consequence of the gravitational instability. In the Weak spin up event, we see that gravitational instability is also triggered in the circumstellar disc around the primary star, but it is cased by the flyby of another star. The gravitational instability in this case is much weaker, and does not lead to further fragmentation and companion formation.

Acknowledgements: RLK acknowledges funding from the Klaus Tschira Foundation. C.F. acknowledges funding by the Australian Research Council (Discovery Projects grant DP230102280), and the Australia-Germany Joint Research Cooperation Scheme (UA-DAAD). C.F. further acknowledges high-performance computing resources provided by the Leibniz Rechenzentrum and the Gauss Centre for Supercomputing (grants pr32lo, pr48pi and GCS Large-scale project 10391), the Australian National Computational Infrastructure (grant ek9) and the Pawsey Supercomputing Centre (project pawsey0810) in the framework of the National Computational Merit Allocation Scheme and the ANU Merit Allocation Scheme. yt (Turk et al. 2011) was used to help visualise and analyse these simulations. The simulation software, FLASH, was in part developed by the Flash Centre for Computational Science at the University of Chicago and the Department of Physics and Astronomy at the University of Rochester.