On the ortho:para Ratio of H3 + in Diffuse Molecular Clouds

The excitation temperature T01 derived from the relative intensities of the J = 0 (para) and J = 1 (ortho) rotational levels of H2 has been assumed to be an accurate measure of the kinetic temperature in interstellar environments. In diffuse molecular clouds, the average value of T01 is ∼70 K. However, the excitation temperature T(H+3) derived from the (J,K) = (1, 1) (para) and (1, 0) (ortho) rotational levels of H+3 has been observed to be ∼30 K in the same types of environments. In this work, we present observations of H+3 in three additional diffuse cloud sight lines for which H2 measurements are available, showing that in four of five cases T01 and T(H+3) are discrepant. We then examine the thermalization mechanisms for the ortho:para ratios of H+3 and H2, concluding that indeed T01 is an accurate measure of the cloud kinetic temperature, while the ortho:para ratio of H+3 need not be thermal. By constructing a steady-state chemical model taking into account the nuclear spin dependence of reactions involving H+3, we show that the ortho:para ratio of H+3 in diffuse molecular clouds is likely governed by a competition between dissociative recombination with electrons and thermalization via reactive collisions with H2. Key words: astrochemistry – ISM: molecules – molecular processes Online-only material: color figures 1