On the relation between diffuse interstellar bands and simple molecular species

We present observations of the major diffuse interstellar bands (DIBs) at 5780 and 5797 Å as well as literature data and our own observations of the violet lines of CH and CH⁺, in the lines of sight toward some 70 stars representing various degrees of the interstellar reddening. The correlations are shown and discussed in the context of indicators such as far-UV extinction parameters and neutral molecular abundances. The results show that the DIBs in question (λλ5797 and 5780) both probably form in diffuse cloud interiors, in a related regime where CH and H₂ form. The ratio of the two DIBs correlates with CH abundance, confirming that the λ5797 carrier is favoured in enhanced molecular gas regions over the λ5780 carrier. The ratio of the two DIBs correlates poorly with CH⁺ abundance. Our compilation of observational data also suggests that the DIB ratio may be equally useful as a cloud type indicator as is R_V, the ratio of total to selective extinction, and much more readily observed.