15min:
INTERSTELLAR H3+ IN METASTABLE ROTATIONAL LEVELS.

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

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). 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, while those in the metastable (3, 3) level do not relax radiatively. 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.