Detection of H 3 + in the Diffuse Interstellar Medium Toward Cygnus OB2 No. 12

Abstract

The molecular ion H 3 + 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, H 3 + has now been observed in the diffuse interstellar medium toward Cygnus OB2 No. 12. Analysis of H 3 + chemistry suggests that the high H 3 + column density (3.8 × 10 14 per square centimeter) is due not to a high H 3 + concentration but to a long absorption path. This and other work demonstrate the ubiquity of H 3 + and its potential as a probe of the physical and chemical conditions in the interstellar medium.