OBSERVATIONS OF $H^{+}_{3}$ IN THE DIFFUSE INTERSTELLAR MEDIUM
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Date
2002
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Publisher
Ohio State University
Abstract
The unexpectedly high column density of $H^{+}_{3}$ $observed^{a,b}$ in the diffuse interstellar medium towards Cygnus OB2 12 presented quite an enigma for interstellar chemistry. Since $H^{+}_{3}$ has now been detected in many $sightlines^{c}$, it is clear that there is a general problem with the chemical models. The standard model of $H^{+}_{3}$ chemistry in diffuse clouds contains only three parameters: $\zeta$ (the $H_{2}$ ionization rate), $k_{e}$ (the $H_{3}$ dissociative recombination rate constant), and $[e]/[H_{2}]$ (the electron fraction) --- evidently, at least one of the assumed values for these parameters is in error by one to two orders of magnitude. There are only three options: (1) the electron fraction is lower than expected from complete conversion of $C \rightarrow C^{+}, (2)$ the value of $k_{e}$ for $J = 1 H^{+}_{3}$ (and cold electrons) is lower than that reported in the laboratory for rotationally hot $H^{+}_{3}$, and/or (3) the value of $\zeta$ is higher in diffuse clouds than in dense clouds due to the presence of low-energy cosmic rays. We report new observations of $H^{+}_{3}$ in the diffuse interstellar medium using CGS4 at UKIRT and IRCS at Subaru, which seem to rule out option (1). In particular, we have tentatively detected $H^{+}_{3}$ in the classical diffuse cloud towards $\zeta$ Per, toward which both $C^{+}$ and $H_{2}$ have been spectroscopically measured. It now seems that it is up to the dissociative recombination community to unambiguously determine the appropriate value of $k_{e}$ through both experiment and theoretical calculation. With a definitive value of $k_{e}$ in hand, observations of $H^{+}_{3}$ will then serve as a direct probe of the cosmic-ray ionization rate $\zeta$ in diffuse clouds.
Description
$^{a}$ B. J. McCall, T. R. Geballe, K. H. Hinkle, and T. Oka, Science 279, 1910 (1998) $^{b}$ T. R. Geballe, B. J. McCall, K. H. Hinkle, and T. Oka, Astrophysical Journal 522, 338 (1999) $^{c}$ B. J. McCall et al. Astrophysical Journal 567, 391 (2002)
Author Institution: Department of Astronomy \& Department of Chemistry, University of California at Berkeley; Department of Astronomy \& Department of Chemistry, Gemini Observatory; Department of Astronomy \& Department of Chemistry, National Optical Astronomy Observatories; Department of Astronomy \& Department of Chemistry, National Astronomical Observatory of Japan; Department of Chemistry and Department of Astronomy \& Astrophysics, University of Chicago
Author Institution: Department of Astronomy \& Department of Chemistry, University of California at Berkeley; Department of Astronomy \& Department of Chemistry, Gemini Observatory; Department of Astronomy \& Department of Chemistry, National Optical Astronomy Observatories; Department of Astronomy \& Department of Chemistry, National Astronomical Observatory of Japan; Department of Chemistry and Department of Astronomy \& Astrophysics, University of Chicago