Conditions for the Formation of First-Star Binaries

The fragmentation process of primordial-gas cores during prestellar collapse is studied using three-dimensional nested-grid hydrodynamics. Starting from the initial central number density of nc ∼ 10 cm, we follow the evolution of rotating spherical cores up to the stellar density nc ≃ 10 cm. An initial condition of the cores is specified by three parameters: the ratios of the rotation and thermal energies to the gravitational energy (β0, and α0, respectively), and the amplitude of the bar-mode density perturbation (Aφ). Cores with rotation β0 > 10 −6 are found to fragment during the collapse. The fragmentation condition hardly depends on either the initial thermal energy α0 or amplitude of bar-mode perturbation Aφ. Since the critical rotation parameter for fragmentation is lower than that expected in the first star formation, binaries or multiples are also common for the first stars. Subject headings: cosmology: theory — galaxies: formation — hydrodynamics— stars: formation