The Formation of Population III Binaries

We explore the possibility for the formation of Population III binaries. Thecollapse of a rotating cylinder is simulated with a three-dimensional,high-resolution nested grid, assuming the thermal history of primordial gas.The simulations are done with dimensionless units, and the results areapplicable to low-mass as well as massive systems by scaling with the initialdensity. We find that if the initial angular momentum is as small as $\beta\approx 0.1$, where $\beta$ is the ratio of centrifugal force to pressureforce, then the runaway collapse of the cloud stops to form arotationally-supported disk. After the accretion of the envelope, the diskundergoes a ring instability, eventually fragmenting into a binary. If theinitial angular momentum is relatively large, a bar-type instability arises,resulting in the collapse into a single star through rapid angular momentumtransfer. The present results show that a significant fraction of Pop III starsare expected to form in binary systems, even if they are quite massive or lessmassive. The cosmological implications of Population III binaries are brieflydiscussed.