Can supermassive stars form in protogalaxies due to internal Lyman-Werner feedback?

TL;DR

This study investigates whether internal Lyman-Werner feedback in atomic-cooling halos can promote supermassive star formation, finding that such feedback is unlikely to enable the rapid formation of supermassive black hole seeds.

Contribution

The paper provides hydrodynamical simulation evidence that internal LW feedback does not significantly promote supermassive star formation in atomic-cooling halos.

Findings

Internal LW feedback reduces H2 and increases temperature but does not promote rapid accretion.

Halo potential wells are too shallow to heat gas sufficiently for supermassive star formation.

Internal LW feedback is unlikely to facilitate supermassive black hole seed formation.

Abstract

Population III stars are possible precursors to early massive and supermassive black holes (BHs). The presence of soft UV Lyman Werner (LW) background radiation can suppress Population III star formation in minihalos and allow them to form in pristine atomic cooling halos. In the absence of molecular hydrogen ($\rm H_2$) cooling, atomic-cooling halos enable rapid collapse with suppressed fragmentation. High background LW fluxes from preceding star-formation have been proposed to dissociate $\rm H_2$. This flux can be supplemented by LW radiation from one or more Population III star(s) in the same halo, reducing the necessary background level. Here we consider atomic-cooling halos in which multiple protostellar cores form close to one another nearly simultaneously. We assess whether the first star's LW radiation can dissociate nearby $\rm H_2$, enabling the prompt formation of a second, supermassive star (SMS) from warm, atomically-cooled gas. We use a set of hydrodynamical simulations with the code ENZO, with identical LW backgrounds centered on a halo with two adjacent collapsing gas clumps. When an additional large local LW flux is introduced, we observe immediate reductions in both the accretion rates and the stellar masses that form within these clumps. While the LW flux reduces the $\text{H}_2$ fraction and increases the gas temperature, the halo core's potential well is too shallow to promptly heat the gas to $\gtrsim$ 1000 K and increase the accretion rate onto the second protostar. We conclude that internal LW feedback inside atomic-cooling halos is unlikely to facilitate the formation of SMSs or massive BH seeds.