INTERSTELLAR $H^{+}_{3}$ IN METASTABLE ROTATIONAL LEVELS
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Date
2003
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Ohio State University
Abstract
A recent tentative detection of interstellar $H^{+}_{3}$ toward the Galactic center has revealed the possible presence of $H_{3}^{+}$ in a metastable rotational level $(J,K) = (3,3).^{a}$ The absence of $H_{3}^{+}$ in the lower $(2, 2)$ and $(2, 1)$ rotational levels during the same observation demonstrates a non-thermal populational distribution in which $H_{3}^{+}$ 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. $H_{3}^{+}$ in the metastable levels thermalize only through reactive collisions with other species, mainly $H_{2}, H, He$ and electrons. We discuss thermalization of $H_{3}^{+}$ 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 $H_{3}^{+}$. Properly interpreted, $H_{3}^{+}$ 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.
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$^{a}$M. Goto, B. J. McCall, T. R. Geballe, T. Usuda, N. Kobayashi, H. Terada and T. Oka, PASJ 54, 951 (2002) $^{b}$J. K. G. Watson, J. Mol. Spectrosc. 40, 536 (1971) $^{c}$F. S. Pan and T. Oka, ApJ 305, 518 (1986)
Author Institution: Department of Chemistry and Department of Astronomy and Astrophysics, the Enrico Fermi Institute, the University of Chicago
Author Institution: Department of Chemistry and Department of Astronomy and Astrophysics, the Enrico Fermi Institute, the University of Chicago