Disk fragmentation and intermittent accretion onto supermassive stars

TL;DR

This study investigates how gas fragmentation and intermittent accretion influence the growth of supermassive stars, showing that their formation can proceed despite ionizing radiation feedback due to short quiescent periods.

Contribution

It demonstrates through simulations that even with disk fragmentation and intermittent accretion, supermassive stars can form without being halted by ionizing radiation feedback.

Findings

Accretion becomes intermittent due to disk instabilities.

Quiescent periods are shorter than Kelvin-Helmholtz timescales.

Ionizing radiation does not prevent supermassive star formation.

Abstract

Supermassive stars (SMSs) with $\sim10^{4-5}~\mathrm{M}_{\odot}$ are candidate objects for the origin of supermassive black holes observed at redshift $z$>6. They are supposed to form in primordial-gas clouds that provide the central stars with gas at a high accretion rate, but their growth may be terminated in the middle due to the stellar ionizing radiation if the accretion is intermittent and its quiescent periods are longer than the Kelvin-Helmholtz (KH) timescales at the stellar surfaces. In this paper, we examine the role of the ionizing radiation feedback based on the accretion history in two possible SMS-forming clouds extracted from cosmological simulations, following their evolution with vertically-integrated two-dimensional hydrodynamic simulations with detailed thermal and chemical models. The consistent treatment of the gas thermal evolution is crucial for obtaining the realistic accretion history, as we demonstrate by performing an additional run with a barotropic equation of state, in which the fluctuation of the accretion rate is artificially suppressed. We find that although the accretion becomes intermittent due to the formation of spiral arms and clumps in gravitationally unstable disks, the quiescent periods are always shorter than the KH timescales, implying that SMSs can form without affected by the ionizing radiation.