ON THE ORTHO:PARA RATIO OF H3+ IN DIFFUSE MOLECULAR CLOUDS

The excitation temperature T-01 derived from the relative intensities of the J = 0 (para) and J = 1 (ortho) rotational levels of H-2 has been assumed to be an accurate measure of the kinetic temperature in interstellar environments. In diffuse molecular clouds, the average value of T-01 is similar to 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 similar to 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 H-2 measurements are available, showing that in four of five cases T-01 and T(H-3(+)) are discrepant. We then examine the thermalization mechanisms for the ortho: para ratios of H-3(+) and H-2, concluding that indeed T-01 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 H-2.