On the relation between diffuse interstellar bands and simple molecular species

We present observations of the major diffuse interstellar bands (DIBs) at 5 780 and 5797 Angstrom as well as literature data and our own observations o f the violet lines of CH and CH+ in the lines of sight toward some 70 stars representing various degrees of the interstellar reddening. The correlatio ns are shown and discussed in the context of indicators such as far-UV exti nction parameters and neutral molecular abundances. The results show that t he DIBs in question (lambda lambda 5797 and 5780) both probably form in dif fuse cloud interiors, in a related regime where CH and H-2 form. The ratio of the two DIBs correlates with CH abundance, confirming that the lambda 57 97 carrier is favoured in enhanced molecular gas regions over the lambda 57 80 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 t o selective extinction, and much more readily observed.