Knowledge Bank

A recent tentative detection of interstellar $H3+$ toward the Galactic center has revealed the possible presence of $H3+$ in a metastable rotational level $(J,K)=(3,3).a$ The absence of $H3+$ in the lower $(2,2)$ and $(2,1)$ rotational levels during the same observation demonstrates a non-thermal populational distribution in which $H3+$ in the two lower levels relax to the lowest $(1,1)$ level by spontaneous emission due to a dipole moment induced by centrifugal $distortion,b$ while those in the metastable $(3,3)$ level do not relax $radiatively.c$ Such metastability is expected to occur also for rotational levels with $J=K=5,6,7$ etc. and a few other levels. $H3+$ in the metastable levels thermalize only through reactive collisions with other species, mainly $H2,H,He$ and electrons. We discuss thermalization of $H3+$ and the resulting non-thermal population in the interstellar medium taking into account both the spontaneous emission and the collision and the production and destruction of $H3+$. Properly interpreted, $H3+$ in metastable rotational levels will provide us with a useful astrophysical probe to measure temperature and density of the interstellar medium with relatively high temperature.