The formation of protostellar binaries in primordial minihalos

Abstract

The first stars are known to form in primordial gas, either in minihalos with about $10^6$~M$_\odot$ or so-called atomic cooling halos of about $10^8$~M$_\odot$. Simulations have shown that gravitational collapse and disk formation in primordial gas yield dense stellar clusters. In this paper, we focus particularly on the formation of protostellar binary systems, and aim to quantify their properties during the early stage of their evolution. For this purpose, we combine the smoothed particle hydrodynamics code GRADSPH with the astrochemistry package KROME. The GRADSPH-KROME framework is employed to investigate the collapse of primordial clouds in the high-density regime, exploring the fragmentation process and the formation of binary systems. We observe a strong dependence of fragmentation on the strength of the turbulent Mach number $\mathcal{M}$ and the rotational support parameter $\beta{}$. Rotating clouds show significant fragmentation, and have produced several Pop.~III proto-binary systems. We report maximum and minimum mass accretion rates of $2.31 \times 10^{-1}$~M$_{\odot}$ yr$^{-1}$ and $2.18\times 10^{-4}$~M$_{\odot}$ yr$^{-1}$. The mass spectrum of the individual Pop III proto-binary components ranges from $0.88$~M$_{\odot}$ to $31.96$~M$_{\odot}$ and has a sensitive dependence on the Mach number $\mathcal{M}$ as well as on the rotational parameter $\beta{}$. We also report a range from $\sim0.01$ to $\sim1$ for the mass ratio of our proto-binary systems.