Infrared and visual interstellar absorption features towards heavily reddened field stars

We present measurements of the 3.4-μm hydrocarbon dust absorption feature, and four visual diffuse interstellar bands, for 12 highly reddened [9.0 < A(V) < 15.8], early-type stars identified from the Stephenson catalogue, covering a range of Galactic longitude. The hydrocarbon feature is detected in 11 sightlines with optical depths of up to 0.05, and corresponding column densities are derived. The feature is weaker per unit A(V) than previously reported, further underlining earlier observations of its enhancement in the Galactic Centre. A comparison of the 3.4-μm feature substructure indicates organic material similar to that seen in earlier diffuse medium studies, suggesting a uniform formation history across the Galactic disc. The profile is well matched by both meteoritic material and several laboratory-prepared analogues. The diffuse interstellar bands (DIBs) measured were λλ6177, 6203, 6284 and 6614; no strong correlations were detected, either between these bands themselves or between the DIBs and the 3.4-μm feature, although λλ6284 and 6614 appear to vary together. If the carriers of the DIBs are organic and molecular in nature and have cosmic carbon abundance requirements similar to those of the C-H stretch, these results imply that there is little direct carbon exchange between them and the aliphatic dust component. Measurements of the extinction to these objects suggest luminosity anomalies similar to that already seen in Cyg OB2 no 12.