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B.J. McCall (U. Chicago), T.R. Geballe (Gemini), K.H. Hinkle (NOAO), T. Oka (U. Chicago)
The H3+ molecular ion has long been considered the cornerstone of gas-phase chemistry in dense clouds. In the dense cloud environment, the H3+ number density is controlled by the competition between formation from cosmic-ray ionization of H2 and destruction by reaction with CO. Observations of the \nu2 vibrational band of H3+ near 3.67 \mum toward several deeply embedded young stellar objects have yielded column densities that are in good agreement with our chemical understanding of molecular clouds.
In the diffuse cloud environment, the destruction of H3+ is dominated not by chemical reaction but by electron recombination. Based on the best available laboratory measurements and a simple model of diffuse cloud chemistry, the H3+ number density in diffuse clouds should be two to three orders of magnitude lower than that in dense clouds.
Despite this low expected number density, H3+ has been detected in the diffuse clouds along the line of sight to Cygnus OB2 No.\ 12 with a column density comparable to that of a dense cloud (3.8~\times 1014~cm-2). This surprising detection suggests either that there is a surprisingly long column (a few hundred parsecs) of absorbing gas along this line of sight, or that there is a problem with our model of diffuse cloud chemistry.
Subsequent observations have shown a similar column density of H3+ along the line of sight to the neighboring source Cygnus OB2 No.\ 5 (~ 3 pc away from No.\ 12), but only upper limits along other sightlines with similar visual extinctions. Although more observations of sources in Cygnus OB2 and elsewhere are needed, these preliminary results suggest that the Cygnus OB2 region may have an unusually high H3+ number density.
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