Program: GS-2009B-Q-71

The ratio between the populations of the two lowest rotational levels of H$_2$, $J=0$ and $J=1$, can be used to determine the temperature of interstellar gas (referred to as $T_{01}$). Likewise, a temperature can be inferred from the populations of the $(J,K)=(1,1)$ and $(J,K)=(1,0)$ states of H$_3^+$. However, the average temperatures derived from these methods ($T_{01}\approx60$~K, $T({\rm H}_3^+)\approx30$~K) do not agree. Theories predict that the deviation from a Boltzmann distribution in H$_3^+$ is due to collisions with H$_2$ which can change the spin alignment. Recent laboratory results confirm this deviation from a thermal distribution, and provide a relationship between the (1,0)/(1,1) ratio of H$_3^+$ and the (1)/(0) ratio of H$_2$. Exploiting this relationship, we plan to use H$_3^+$ observations as a proxy in determining $T_{01}$ in highly extincted sight lines where H$_2$ cannot be observed in the UV. To validate this method though, we propose to observe H$_3^+$ in sight lines where H$_2$ measurements have already been made.