Contributed Talk

The Deep 3.4 mm Interstellar Absorption Feature toward the IRAS 18511+0146 Cluster

¹Marie Godard, ²Tom Geballe, ³Guillermo Munoz Caro, ⁴Emmanuel Dartois (¹NASA Godard Space Flight Center, Institut d'Astrophysique Spatiale, ²Gemini Observatory, ³Centro de astrobiologia (CSIC-INTA), ⁴Institut d'Astrophysique Spatiale)

An absorption feature at 3.4 mm, widely observed in the diffuse interstellar medium, is produced by aliphatic hydrocarbon dust. In the local interstellar medium, there is usually a linear correlation between the 3.4 mm optical depth and the visual extinction Av. However, the 3.4 mm 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/t_3.4 in the Galactic disk.

In order to verify the unusually high 3.4 mm optical depth toward IRAS 18511+0146 and test for patchiness in the foreground diffuse cloud material, we observed several sources of the cluster associated with IRAS 18511+0146. The 3-4 mm spectra have been obtained for three of these sources with the near-infrared imager/spectrograph (NIRI) at the Gemini North telescope. After subtraction of the superimposed 3.47 mm band, produced in dense interstellar gas associated with the cluster, we have determined the optical depths of the 3.4 mm absorption for the three lines of sight. We discuss the distance and extinction to the IRAS 18511+0146 cluster and possible interpretations of the obtained high optical depth of the 3.4 mm absorption. The high t_3.4 makes this cluster a unique and valuable line of sight for studying the Galactic hydrocarbon dust.

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