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 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.
