Diffuse interstellar bands in the Taurus dark clouds

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Abstract
Strengths and profiles of seven diffuse interstellar bands have been measured in a group of field stars situated behind the Taurus dark cloud complex. Comparison with current ‘band family’ theories shows that, within a single cloud complex, the relative proportions of different family carriers can vary in rather close unison: although correlations with extinction are in all cases poor, some pairs of bands are nonetheless strongly interrelated. Band strengths relative to reddening are investigated and found to be reasonably well fitted by a model in which all of the DIB production takes place in a surface layer of the cloud (about 0.7 mag thick in Av). These results are compared with existing data for the ρ Oph cloud. Band production efficiency varies in the same way in both regions, falling rapidly with increasing |$E(B-V)$| to a limiting value a factor of 3–10 below the norm for the diffuse ISM. However, comparison of grain properties and the UV radiation field in the two cases shows that the relative weakness of the diffuse bands in dark clouds is driven by changing grain properties rather than by suppression of the UV radiation field. This argues against an origin in ionized PAH species or other ionized molecules. Models involving pre-ionization or pre-dissociation transitions in gas-phase carriers also seem to be excluded. Systematic variation of the band profiles in the highest-extinction object in the sample (HD 283809) is consistent with the formation of at least some of the diffuse bands in impurity centres in mantled grains.

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