Formation of supermassive stars in the first star clusters

TL;DR

This paper demonstrates through simulations that supermassive stars of 10^3 to 10^4 solar masses can form in early star clusters via collisions and accretion, even with small-scale fragmentation.

Contribution

It introduces a combined N-body and hydrodynamical simulation approach to study supermassive star formation in primordial star clusters with small-scale fragmentation.

Findings

Supermassive stars of 10^3-10^4 solar masses form in simulated clusters.

Supermassive stars often exist in binary systems.

Clusters contain smaller stars with masses between 1 and 100 solar masses.

Abstract

The formation of supermassive stars is believed to be an essential intermediate step for the formation of the massive black hole seeds that become the supermassive black holes powering the quasars observed in the early Universe. Numerical simulations have shown that supermassive stars can form in atomic-cooling halos when protostars reach accretion rates higher than $\sim10^{-2}$ M$_\odot$ yr$^{-1}$ and fragmentation is suppressed on pc scales. It is however still uncertain if a supermassive star still emerges when fragmentation occurs at smaller scales and a cluster of stars is formed instead. In this work we explore the problem of massive object formation due to the interplay of collisions and accretion in star clusters at low metallicity. We model a small embedded cluster of accreting protostars following sub-parsec scale fragmentation during the collapse of a primordial gas cloud and follow its evolution by performing $N$-body plus hydrodynamical simulations. Our results show that supermassive stars with 10$^3$ and 10$^4$ M$_\odot$ are always formed due to the interplay of collisions and accretion, and in some cases these objects are part of a binary system. The resulting supermassive star is surrounded by tens of smaller stars with typical masses in the range $1$-$100$ M$_\odot$.