Does the chemothermal instability have any role in the fragmentation of primordial gas

TL;DR

This study investigates whether chemothermal instability drives fragmentation in primordial gas during early star formation, concluding that heating and cooling balance, rather than chemothermal instability, influences fragmentation.

Contribution

The paper provides a detailed 3D simulation analysis showing that fragmentation is driven by the interplay of H2 cooling and heating, not by chemothermal instability.

Findings

Cooling rates are less than heating rates after including compressional heating.

Fragmentation is caused by H2 cooling and heating balance, not chemothermal instability.

Long-term evolution of the disk shows fragmentation due to thermal structure.

Abstract

The collapse of the primordial gas in the density regime $\sim 10^{8}\hbox{--}10^{10}$ cm$^{-3}$ is controlled by the three-body $\rm H_2$ formation process, in which the gas can cool faster than free-fall time $\hbox{--}$ a condition proposed as the chemothermal instability. We investigate how the heating and cooling rates are affected during the rapid transformation of atomic to molecular hydrogen. With a detailed study of the heating and cooling balance in a 3D simulation of Pop~III collapse, we follow the chemical and thermal evolution of the primordial gas in two dark matter minihaloes. The inclusion of sink particles in modified Gadget-2 smoothed particle hydrodynamics code allows us to investigate the long term evolution of the disk that fragments into several clumps. We find that the sum of all the cooling rates is less than the total heating rate after including the contribution from the compressional heating ($pdV$). The increasing cooling rate during the rapid increase of the molecular fraction is offset by the unavoidable heating due to gas contraction. We conclude that fragmentation occurs because $\rm H_2$ cooling, the heating due to $\rm H_2$ formation and compressional heating together set a density and temperature structure in the disk that favors fragmentation, not the chemothermal instability.