Making a supermassive star by stellar bombardment

TL;DR

This paper explores how stellar collisions during the collapse of gas clouds can enable the formation of supermassive stars, which are potential precursors to early supermassive black holes, by preventing protostar contraction and promoting growth.

Contribution

It introduces a semianalytic model showing that high-density gas fragmentation and frequent stellar mergers can lead to the formation of supermassive stars in the early universe.

Findings

Frequent stellar mergers prevent protostar contraction at high densities.

Low-density fragmentation leads to smaller objects (~10^4 solar masses).

High-density fragmentation allows growth of supermassive stars (~10^5-10^6 solar masses).

Abstract

Approximately two hundred supermassive black holes (SMBHs) have been discovered within the first $\sim$Gyr after the Big Bang. One pathway for the formation of SMBHs is through the collapse of supermassive stars (SMSs). A possible obstacle to this scenario is that the collapsing gas fragments and forms a cluster of main-sequence stars. Here we raise the possibility that stellar collisions may be sufficiently frequent and energetic to inhibit the contraction of the massive protostar, avoiding strong UV radiation driven outflows, and allowing it to continue growing into an SMS. We investigate this scenario with semianalytic models incorporating star formation, gas accretion, dynamical friction from stars and gas, stellar collisions, and gas ejection. We find that when the collapsing gas fragments at a density of $\lesssim 3\times 10^{10}\,\mathrm{cm^{-3}}$, the central protostar contracts due to infrequent stellar mergers, and in turn photoevaporates the remaining collapsing gas, resulting in the formation of a $\lesssim 10^4~{\rm M_\odot}$ object. On the other hand, when the collapsing gas fragments at higher densities (expected for a metal-poor cloud with $Z\lesssim10^{-5}\,{\rm Z_\odot}$ with suppressed ${\rm H_2}$ abundance) the central protostar avoids contraction and keeps growing via frequent stellar mergers, reaching masses as high as $\sim 10^5-10^6\,{\rm M_\odot}$. We conclude that frequent stellar mergers represent a possible pathway to form massive BHs in the early universe.