H₃⁺ in diffuse interstellar clouds: A tracer for the cosmic-ray ionization rate

Using high-resolution infrared spectroscopy we have surveyed twenty sight lines for H₃⁺ absorption. H₃⁺ is detected in eight diffuse cloud sight lines with column densities varying from 0.6 × 10¹⁴ to 3.9 × 10¹⁴ cm^-2. This brings to 14 the total number of diffuse cloud sight lines where H ₃⁺ has been detected. These detections are mostly along sight lines concentrated in the Galactic plane but are well dispersed in Galactic longitude. The results imply that abundant H₃⁺ is common in the diffuse interstellar medium. Because of the simple chemistry associated with H₃⁺ production and destruction, these column density measurements can be used in concert with various other data to infer the primary cosmic-ray ionization rate, ζ_p. Values range from 0.5 × 10^-16 to 3 × 10^-16 s^_1 with an average of 2 × 10^-16 s^_1. Where H ₃⁺ is not detected, the upper limits on the ionization rate are consistent with this range. The average value of ζ_p is about 1 order of magnitude larger than both the canonical rate and rates previously reported by other groups using measurements of OH and HD. The discrepancy is most likely due to inaccurate measurements of rate constants and the omission of effects which were unknown when those studies were performed. We believe that the observed column density of H₃⁺ is the most direct tracer for the cosmic-ray ionization rate due to its simple chemistry. Recent models of diffuse cloud chemistry require cosmic-ray ionization rates on the order of 10^-16 s^_1 to reproduce observed abundances of various atomic and molecular species, in rough accord with our observational findings.