On the operation of the chemothermal instability in primordial star-forming clouds

TL;DR

This study uses 3D simulations to explore how chemothermal instability influences fragmentation in primordial star-forming clouds, revealing conditions that lead to early gravitational collapse and clump formation.

Contribution

It provides new insights into the role of chemothermal instability in primordial clouds and highlights the potential for early fragmentation during initial collapse phases.

Findings

Chemothermal instability causes a dip in cooling time, promoting fragmentation.

Secondary clumps can form and potentially collapse independently.

Fragmentation likelihood varies among different primordial haloes.

Abstract

We investigate the operation of the chemothermal instability in primordial star-forming clouds with a suite of three-dimensional, moving-mesh simulations. In line with previous studies, we find that the gas at the centre of high-redshift minihaloes becomes chemothermally unstable as three-body reactions convert the atomic hydrogen into a fully molecular gas. The competition between the increasing rate at which the gas cools and the increasing optical depth to H2 line emission creates a characteristic dip in the cooling time over the free-fall time on a scale of 100 au. As a result, the free-fall time decreases to below the sound-crossing time, and the cloud may become gravitationally unstable and fragment on a scale of a few tens of au during the initial free-fall phase. In three of the nine haloes investigated, secondary clumps condense out of the parent cloud, which will likely collapse in their own right before they are accreted by the primary clump. In the other haloes, fragmentation at such an early stage is less likely. However, given that previous simulations have shown that the infall velocity decreases substantially once the gas becomes rotationally supported, the amount of time available for perturbations to develop may be much greater than is evident from the limited period of time simulated here.