15min:

TAKESHI OKA, ERIK EPP, Department of Chemistry and Department of Astronomy and Astrophysics, the Enrico Fermi Institute, the University of Chicago, Chicago, IL 60637.

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 3 ^, has demonstrated a remarkable non-thermal rotational distribution of H₃⁺. In cloud components which accommodates high H₃⁺ column density on the order of 3×10¹⁵ cm^-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) (1,1) with the lifetime of 27.2 days due to centrifugal distortion induced dipole moment and the metastability of the (3,3) level from which spontaneous emission is forbidden.

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 Tennyson 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) 1 and n(3,3)/n(2,2)\geq5 indicate an environment with high cloud temperature of T\geq300 K and low density n(H₂)\leq50 cm^-3. They also suggest that observations of H₃⁺ in higher metastable levels are realistic.