15min:

B. J. MCCALL, Department of Astronomy & Department of Chemistry, University of California at Berkeley, 601 Campbell Hall, Berkeley, CA 94720; T. R. GEBALLE, Gemini Observatory, Hilo, HI 96720; K. H. HINKLE, National Optical Astronomy Observatories, Tucson, AZ 85726; M. GOTO, N. KOBAYASHI, H. TERADA, T. USUDA, National Astronomical Observatory of Japan, Hilo, HI 96720; T. OKA, Department of Chemistry and Department of Astronomy & Astrophysics, University of Chicago, Chicago, IL 60637.

\hspace0.3in The unexpectedly high column density of H₃⁺ observed^, in the diffuse interstellar medium towards Cygnus OB2 12 presented quite an enigma for interstellar chemistry. Since H₃⁺ has now been detected in many sightlines, it is clear that there is a general problem with the chemical models.

\hspace0.3in The standard model of H₃⁺ chemistry in diffuse clouds contains only three parameters: (the H₂ ionization rate), k_e (the H₃⁺ dissociative recombination rate constant), and [e]/[ H₂] (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 C⁺, (2) the value of k_e for J=1 H₃⁺ (and cold electrons) is lower than that reported in the laboratory for rotationally hot H₃⁺, and/or (3) the value of is higher in diffuse clouds than in dense clouds due to the presence of low-energy cosmic rays.

\hspace0.3in We report new observations of H₃⁺ 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 H₃⁺ in the classical diffuse cloud towards ~Per, toward which both C⁺ and H₂ have been spectroscopically measured.

\hspace0.3in 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 H₃⁺ will then serve as a direct probe of the cosmic-ray ionization rate in diffuse clouds.