Knowledge Bank

In diffuse molecular clouds (environments with high molecular fraction, but low CO abundance), the relative populations of the $J=0$ (\textit{para}) and $J=1$ (\textit{ortho}) rotational levels of H$2$ are often used as a measure of the cloud kinetic temperature, $T01$. Typically, $T01$ is on the order of 50-70 K, but in similar environments, the excitation temperature $T$(H$\mathrm{3+}$) derived from the $(J,K)$ = (1,0) (\textit{ortho}) and (1,1) (\textit{para}) rotational levels of H$\mathrm{3+}$ is 20-40 K. We have extended the number of sight lines in which both $T01$ and $T$(H$\mathrm{3+}$) have been measured from two to five, and in four of the five cases, the two temperatures are discrepant in the same cloud. Using a steady state chemical model based on rate coefficients calculated with a microcanonical statistical approach, we find that the discrepancy between $T01$ and $T$(H$\mathrm{3+}$) likely arises from incomplete thermalization caused by competition between the thermalization reaction H$\mathrm{3+}$ + H$2$ $\to$ H$2$ + H$\mathrm{3+}$ and dissociative recombination of H$\mathrm{3+}$ with electrons.