On the Ortho:Para Ratio of H3+ in Diffuse Molecular Clouds

TL;DR

This paper investigates the ortho:para ratio of H3+ in diffuse molecular clouds, revealing it is not always thermal and is influenced by chemical reactions, contrasting with the temperature derived from H2 measurements.

Contribution

The study provides new observations of H3+ in multiple diffuse clouds and develops a chemical model explaining the non-thermal ortho:para ratio of H3+ in these environments.

Findings

T_01 accurately measures kinetic temperature in diffuse clouds.

H3+ ortho:para ratio often deviates from thermal equilibrium.

Chemical reactions control H3+ ortho:para ratio in diffuse clouds.

Abstract

The excitation temperature T_01 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 T_01 is ~70 K. However, the excitation temperature T(H3+) derived from the (J,K) = (1,1) (para) and (1,0) (ortho) rotational levels of H3+ has been observed to be ~30 K in the same types of environments. In this work, we present observations of H3+ in three additional diffuse cloud sight lines for which H2 measurements are available, showing that in 4 of 5 cases T_01 and T(H3+) are discrepant. We then examine the thermalization mechanisms for the ortho:para ratios of H3+ and H2, concluding that indeed T_01 is an accurate measure of the cloud kinetic temperature, while the ortho:para ratio of H3+ need not be thermal. By constructing a steady-state chemical model taking into account the nuclear-spindependence of reactions involving H3+, we show that the ortho:para ratio of H3+ in diffuse molecular clouds is likely governed by a competition between dissociative recombination with electrons and thermalization via reactive collisions with H2.