Knowledge Bank

The recent detection of the $R(3,3)l$ absorption line of $H3+$ toward the Galactic center infrared sources GCS 3-2 and GC IRS $3a,b$ has demonstrated a remarkable non-thermal rotational distribution of $H3+$. In cloud components which accommodates high $H3+$ column density on the order of $3\times 1015cm-2$, the $(J,K)=(3,3)$ level which is 361.5 K above the lowest (1,1) level is populated with a comparable abundance as the (1,1) level, while $H3+$ in the (2,2) level only 151.3 K above is not detectable. This highly non-thermal distribution is the result of a fast spontaneous emission $(2,2)\to (1,1)$ with the lifetime of 27.2 days due to centrifugal distortion induced dipole $momentc$ and the metastability of the (3,3) level from which spontaneous emission is $forbidden.d$ In order to understand the observed non-thermal distribution and to plan strategy for observing $H3+$ in higher metastable rotational levels (4,4), (5,5) and (6,6), we have conducted a model calculation on the $H3+$ equilibration in which spontaneous emissions and collision-induced rotational transitions are treated with steady state approximation. Accurately calculated Einstein coefficients of Neale, Miller and $Tennysone$ are used. Rates of collision-induced rotational transitions were estimated on the assumption of complete random collisional selection rules based only on the principle of detailed balancing. The results show that the preliminary observed values of $n(3,3)/n(1,1)\sim 1$ and $n(3,3)/n(2,2)\ge 5$ indicate an environment with high cloud temperature of $T\ge 300K$ and low density $n(H2)\le 50cm-3$. They also suggest that observations of $H3+$ in higher metastable levels are realistic.