1996A&A...308..908K

The paper considers the properties of H I clouds seen in front of the two "arms" of the giant CO complex observed in Perseus (Per OB2 association - see Fig. 1). The clouds observed towards both "arms" show striking differences in the pattern of interstellar absorptions: the shapes of the extinction curves derived from vacuum-UV photometric and spectroscopic data, the strength ratio of the two major diffuse interstellar bands: 5780 and 5797 and, probably in the strengths of molecular features. The diffuse band ratio which varies across the Per OB2 association suggests, together with the varying widths of sodium lines, that two, partially overlapping clouds, differing strongly in the optical properties, are situated towards the aggregate. Only towards one of the "arms" we observe the absorption features of simple, interstellar molecules such as CN or CH. The results suggest that at least the peripheral layers of giant molecular clouds, giving birth to OB associations, are not well-mixed, containing thus H I clouds of different provenience. The hypothesis that giant clouds are formed by gathering diffuse interstellar clouds gets support.