On the ortho-H2/para-H2 ratio in molecular clouds

The value of the ortho-H₂/para-H₂ ratio is relevant to discussions of collisional excitation processes in interstellar clouds. We consider the temporal evolution of the relative abundance of the ortho- and para-modifications of H₂. Owing to the proton interchange process, H₂ (ortho) + H⁺ ⇌ H₂ (para) + H⁺ + 170.5 K thermal equilibrium is attained over a time-scale, τ, which is determined by the proton density. In clouds in which the molecular hydrogen density, $$10^2\lesssim n(\text H_2)\lesssim 10^3 \text {cm}^{-3}$$, where photoprocesses are important, the proton density remains high and $$\tau \approx 10^6\enspace \text yr$$. In denser clouds, $$10^4\lesssim n(\text H_2)\lesssim 10^5 \text {cm}^{-3}$$, the proton density is reduced by charge exchange reactions with neutral molecules, and $$\tau \approx 10^7\enspace \text yr$$, which is comparable with the probable cloud lifetime. The value of the ortho:para ratio in a given cloud will depend on its chemical history.