Binary formation in star-forming clouds with various metallicities

Cloud evolution for various metallicities is investigated by three-dimensional nested grid simulations, in which the initial ratio of rotational to gravitational energy of the host cloud β₀ ( = 10^-1 to 10^-6) and cloud metallicity Z ( = 0-Z⊙) are parameters. Starting from a central number density of n_c = 10 ⁴ cm^-3, cloud evolution for 48 models is calculated until the protostar is formed (nc ∼ 10²³ cm^-3) or fragmentation occurs. The fragmentation condition depends on both the initial rotational energy and the cloud metallicity. Cloud rotation promotes fragmentation, while fragmentation tends to be suppressed in clouds with higher metallicity. Fragmentation occurs when ;β₀ ≥ 10^-3 in clouds with solar metallicity (Z = Z⊙), while fragmentation occurs when β₀> 10^-5 in the primordial gas cloud (Z = 0). Clouds with lower metallicity have larger probability of fragmentation, indicating that the binary frequency is a decreasing function of cloud metallicity. Thus, the binary frequency at the early universe (or lower metallicity environment) is higher than at the present day (or higher metallicity environment). In addition, binary stars born from low-metallicity clouds have shorter orbital periods than those from highmetallicity clouds. These trends are explained in terms of the thermal history of the collapsing cloud.