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Context. An absorption feature at 3.4 μm, widely observed in the diffuse interstellar medium (ISM), is produced by aliphatic hydrocarbon dust. In the local ISM there is in general a linear correlation between the 3.4 μm optical depth and the visual extinction AV . However, the 3.4 μm optical depth toward the young stellar object IRAS 18511+0146, the highest found in the local diffuse ISM apart from the Galactic centre lines of sight, is three times higher than predicted from the most likely value for the distance of the object, the extinction-distance relation for the Galactic diffuse ISM, and the value of AV /τ3.4 in the Galactic disk. Aims. IRAS 18511+0146 is associated with a small cluster of stars. We have obtained spectra of it as well as two other cluster members in order to verify the unusually high 3.4 μm optical depth toward IRAS 18511+0146 and test for patchiness in the foreground diffuse cloud material. Methods. Spectra covering 2.9–4.1 μm were recorded using the near-infrared imager/spectrograph (NIRI) at the Gemini North telescope. Results. After subtraction of the superimposed 3.47 μm band, produced in dense interstellar gas associated with the cluster, we have determined optical depths of 0.06–0.08 for the 3.4 μm absorption on the three lines of sight. We discuss the distance and extinction to the IRAS 18511+0146 cluster and possible interpretations of the high optical depth of the 3.4 μm absorption. Conclusions. The most probable distance to the IRAS 18511+0146 cluster is 3.9 kpc. Two different interpretations of the high observed τ3.4 are then possible. (i) The AV to distance ratio toward the cluster is approximately the one observed on average in the Galactic diffuse ISM, in which case AV ∼ 7a ndτ3.4 in this direction is three times higher than predicted by the usual AV /τ3.4 observed in local diffuse ISM. (ii) The visual extinction in the direction of the IRAS 18511+0146 cluster is higher (i.e. 15–20 mag), probably due to a diffuse cloud associated with the cluster, resulting in a high value of τ3.4. In either case, the high τ3.4 makes this cluster a unique and valuable line of sight for studying the Galactic hydrocarbon dust.

The deep 3.4 μm interstellar absorption feature toward the IRAS 18511+0146 cluster