Conditions for the Formation of First-Star Binaries

TL;DR

This study uses 3D hydrodynamics simulations to identify conditions under which primordial gas cores fragment, leading to the formation of the first star binaries, highlighting the role of rotation in this process.

Contribution

It provides the first detailed analysis of primordial core fragmentation conditions, emphasizing the importance of rotation over thermal energy or perturbations.

Findings

Primordial cores with rotation parameter eta_0 > 10^{-6} fragment during collapse.

Fragmentation is largely independent of initial thermal energy lpha_0 or bar-mode perturbation amplitude.

Binaries or multiples are common in first star formation due to low critical rotation threshold.

Abstract

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 n \sim10^3 cm^-3, we follow the evolution of rotating spherical cores up to the stellar density n \simeq 10^{22} cm^-3. An initial condition of the cores is specified by three parameters: the ratios of the rotation and thermal energies to the gravitational energy (\beta_0, and \alpha_0, respectively), and the amplitude of the bar-mode density perturbation (A_\phi). Cores with rotation \beta_0 > 10^{-6} are found to fragment during the collapse. The fragmentation condition hardly depends on either the initial thermal energy \alpha_0 or amplitude of bar-mode perturbation A_\phi. 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.