On the behavior of interstellar CO and CN molecules in individual clouds

We discuss the behaviour of the absorption spectral features originating in interstellar biatomic, carbon bearing, molecules situated inside HI clouds; especially these of the CN and CO. They are seemingly related to the same physical properties of interstellar clouds which facilitate the formation or preservation of the carriers of some (narrow) unidentified diffuse interstellar bands (DIBs) and which apparently lead to the far-UV raise of the interstellar extinction curve, typically attributed to the presence of small, dust particles, in the intervening interstellar clouds. All the above mentioned features form the absorption spectra of individual clouds - apparently dependent on the physical parameters of these clouds (Krelowski et al. 1992). The recent review (Krelowski & Sneden, 1995) proposed a spectral classification of HI clouds. We present an investigation of the CN radical, observed only in some special clouds tin the CFHT spectral together with the vacuum-UV spectral features of the CO molecule, well seen in the Hubble Space Telescope high resolution spectra, as well as in very numerous IUE high resolution ones. The behaviour of the spectral features of both molecules is proved to be surprisingly similar and related to that of diffuse interstellar bands.