Detection of H₃⁺ in the diffuse interstellar medium: The galactic center and cygnus OB2 number 12

Absorption lines of H₃⁺ have been detected in the spectra of two infrared sources in the Galactic center and also toward the heavily reddened star Cygnus OB2 No. 12, whose line of sight is believed to include only diffuse interstellar gas. The absorptions toward the Galactic center sources (IRS 3 and GCS 3-2) probably are due to H₃⁺ both in diffuse gas and in molecular clouds. The ratios of H₃⁺ line equivalent width to extinction toward these three sources are much greater than those toward dense clouds where H₃⁺ has been detected previously. Analysis of the spectra coupled with a simple model for the abundance of H₃⁺ in the diffuse interstellar medium implies that the observed H₃⁺ is present at low densities along long path lengths. These are the first detections of H₃⁺ in the diffuse interstellar medium.