Binary Formation in Star-forming Clouds with Various Metallicities

Abstract

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 (p10 1 to 10 6) and cloud metallicity b 0 Z (p0-) are parameters. Starting from a central number density of cm 3 , cloud evolution for 48 4 Z n p 10 , c models is calculated until the protostar is formed (cm 3) or fragmentation occurs. The fragmentation 23 n 10 c 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 in clouds with solar metallicity (), while fragmentation occurs when in the 3 5 b 1 10 Z p Z b 1 10 0 , 0 primordial gas cloud (). Clouds with lower metallicity have larger probability of fragmentation, indicating Z p 0 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 high-metallicity clouds. These trends are explained in terms of the thermal history of the collapsing cloud.