Detection of in the Diffuse Interstellar Medium: the Galactic Center

Absorption lines of have been detected in the spectra of two infrared sources in the Galactic 3 center and also toward the heavily reddened star Cygnus OB2 No. 12, whose line of sight is believed to include only di†use interstellar gas. The absorptions toward the Galactic center sources (IRS 3 and GCS 3-2) probably are due to both in di†use gas and in molecular clouds. The ratios of line equiva3 3 lent width to extinction toward these three sources are much greater than those toward dense clouds where has been detected previously. Analysis of the spectra coupled with a simple model for the 3 abundance of in the di†use interstellar medium implies that the observed is present at low den3 3 sities along long path lengths. These are the Ðrst detections of in the di†use interstellar medium. 3 Subject headings : Galaxy : center È infrared : ISM: lines and bands È ISM: clouds È ISM: molecules È molecular processes È stars : individual (Cygnus OB2 No. 12)