SIMBAD references

We present a survey of diffuse interstellar bands (DIBs) in single clouds. High resolution and high S/N observations of DIBs towards lines of sight representing very different environments allow to investigate mutual correlations in a large consistent sample of DIBs. The classification of DIBs into families is reviewed and refined according to these results. We find that the selected 44 DIBs are due to different carriers. Two isolated families of DIBs (λ5797 et al., λ4501 et al.) are found, which are mutually slightly anticorrelated. The behaviour of DIBs with respect to the local UV field is investigated using hydrogen column densities to estimate the strength of the UV field, resulting in the hypothesis that most of the DIB carriers are molecules sensitive to photo-ionization. With this interpretation, the apparent division of interstellar clouds into ζ, σ and Orion types can be understood as a sequence in strength of the UV field. Differences in strength of the UV field between different lines-of-sight are due to the so-called ``skin-effect'', the effective shielding of UV radiation by the outer layers of interstellar clouds.