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[7.20] H₃⁺ in the diffuse gas in in front of HD183143

T.R. Geballe (Gemini-N), B.J. McCall, T. Oka (U. Chicago), K.H. Hinkle (NOAO)

We have detected absorption lines of H₃⁺ near 3.7~\mum toward the star HD 183143, which is obscured by about five visual magnitudes and has very strong diffuse interstellar bands. Previously, H₃⁺ in the diffuse interstellar medium has been sought and found only toward the galactic center and Cygnus OB2 No. 12 and No. 5. The unexpectedly strong absorption lines toward Cygnus OB2 indicate an order of magnitude more H₃⁺ than expected, using the best available values for the cosmic ray ionization rate and the rate coefficient for dissociative recombination of H₃⁺ on electrons, together with conventional models for the Cygnus OB2 clouds. Specialized models of these clouds have been proposed to account for the larger than expected abundance of H₃⁺. However, the large column density of H₃⁺ observed toward HD183143, a cooler, lower luminosity, less obscured, and less active star than Cygnus OB2 No. 12, coupled with millimeter wave measurements showing little or no associated dense molecular gas, cannot be explained by such models. We discuss other possible explanations for the surprisingly strong H₃⁺ absorption lines in the diffuse interstellar medium.

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