Detection of H₃⁺ in the diffuse interstellar medium toward Cygnus OB2 No. 12

The molecular ion H₃⁺ is considered the cornerstone of interstellar chemistry because it initiates the reactions responsible for the production of many larger molecules. Recently discovered in dense molecular clouds, H₃⁺ has now been observed in the diffuse interstellar medium toward Cygnus OBS No. 12. Analysis of H₃⁺ chemistry suggests that the high H₃⁺ column density (3.8 x 10¹4 per square centimeter) is due not to a high H₃⁺ concentration but to a long absorption path. This and other work demonstrate the ubiquity of H₃⁺ and its potential as a probe of the physical and chemical conditions in the interstellar medium.