Strength Ratios of Diffuse Interstellar Bands in Slightly Reddened Objects

The disk of the Milky Way fills the interstellar medium in the form of discrete clouds, many (∼30) light-years across. The average density of this medium is 1 hydrogen atom per cm3 (Oort limit), in the clouds—several dozen atoms, and between the clouds about 0.01 atoms per cm3. It is well documented that physical properties of individual interstellar clouds are evidently different using high-resolution spectroscopic observations of slightly reddened stars. We prove here that the 5780/5797 strength ratio is nearly constant for all slightly reddened targets. The reason for this phenomenon remains unknown. All optically thin clouds are apparently of σ-type. The question of at which value of color excess one may expect a ζ-type cloud remains unanswered. For some (unknown) reason ζ-type clouds are always relatively opaque and contain a lot of molecular species. In all slightly reddened objects we always observe σ-type intervening clouds, almost free of simple molecules.