NUCLEAR SPIN OF H$_3^+$ IN DIFFUSE MOLECULAR CLOUDS

Abstract

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, $T_{01}$. Typically, $T_{01}$ is on the order of 50-70 K, but in similar environments, the excitation temperature $T$(H$_3^+$) derived from the $(J,K)$ = (1,0) (\textit{ortho}) and (1,1) (\textit{para}) rotational levels of H$_3^+$ is 20-40 K. We have extended the number of sight lines in which both $T_{01}$ and $T$(H$_3^+$) 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 $T_{01}$ and $T$(H$_3^+$) likely arises from incomplete thermalization caused by competition between the thermalization reaction H$_3^+$ + H$_2$ $\to$ H$_2$ + H$_3^+$ and dissociative recombination of H$_3^+$ with electrons.