DETECTION OF H-3(-MEDIUM TOWARD CYGNUS-OB2 NO.12() IN THE DIFFUSE INTERSTELLAR)

The molecular ion H-3(+) is considered the cornerstone of interstellar chemistry because it initiates the reactions responsible for the prod uction of many larger molecules, Recently discovered in dense molecula r clouds. H-3(+) has now been observed in the diffuse interstellar med ium toward Cygnus OB2 No. 12, Analysis of H-3(+) chemistry suggests th at the high H-3(+) column density (3.8 x 10(14) per square centimeter) is due not to a high H-3(+) concentration but to a long absorption pa th. This and other work demonstrate the ubiquity of H-3(+) and its pot ential as a probe of the physical and chemical conditions in the inter stellar medium.