Is H⁺₃cooling ever important in primordial gas?

Studies of the formation of metal-free Population III stars usually focus primarily on the role played by H 2 cooling, on account of its large chemical abundance relative to other possible molecular or ionic coolants. However, while H 2 is generally the most important coolant at low gas densities, it is not an effective coolant at high gas densities, owing to the low critical density at which it reaches local thermodynamic equilibrium (LTE) and to the large opacities that develop in its emission lines. It is therefore possible that emission from other chemical species may play an important role in cooling high-density primordial gas.