Formation of the first star clusters and massive star binaries by fragmentation of filamentary primordial gas clouds

TL;DR

This study uses cosmological simulations to show how filamentary primordial gas clouds fragment and form massive star clusters with binaries, potentially leading to black hole mergers detectable by gravitational wave observatories.

Contribution

It demonstrates a detailed formation pathway for early star clusters and massive binaries from primordial gas cloud fragmentation, incorporating baryonic streaming motions and advanced simulation techniques.

Findings

Primordial gas clouds fragment into multiple massive clumps.

Massive stars formed are in the range of 50-120 solar masses.

Massive star binaries can form within a few million years.

Abstract

We perform a set of cosmological simulations of early structure formation with incorporating baryonic streaming motions. We present a case where a significantly elongated gas cloud with $\sim\!10^4\,$solar masses (${\rm M_\odot}$) is formed in a pre-galactic ($\sim\!10^7\,{\rm M_\odot}$) dark halo. The gas streaming into the halo compresses and heats the massive filamentary cloud to a temperature of $\sim\!10,000\,$Kelvin. The gas cloud cools rapidly by atomic hydrogen cooling, and then by molecular hydrogen cooling down to $\sim\!400\,$Kelvin. The rapid decrease of the temperature and hence of the Jeans mass triggers fragmentation of the filament to yield multiple gas clumps with a few hundred solar masses. We estimate the mass of the primordial star formed in each fragment by adopting an analytic model based on a large set of radiation hydrodynamics simulations of protostellar evolution. The resulting stellar masses are in the range of $\sim\!50$-$120\,{\rm M_\odot}$. The massive stars gravitationally attract each other and form a compact star cluster. We follow the dynamics of the star cluster using a hybrid $N$-body simulation. We show that massive star binaries are formed in a few million years through multi-body interactions at the cluster center. The eventual formation of the remnant black holes will leave a massive black hole binary, which can be a progenitor of strong gravitational wave sources similar to those recently detected by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO).