Astron. Astrophys. 337, 261-274 (1998)

ISO-SWS observations of infrared absorption bands of the diffuse interstellar medium: The 6.2 µm feature of aromatic compounds ^*

W.A. Schutte ¹, K.A. van der Hucht ², D.C.B. Whittet ³, A.C.A. Boogert ⁴, A.G.G.M. Tielens ⁴, P.W. Morris ⁵, J.M. Greenberg ¹, P.M. Williams ⁶, E.F. van Dishoeck ¹, J.E. Chiar ⁷ and Th. de Graauw <sup>8

1</sup> Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, The Netherlands
² Space Research Organization Netherlands, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
³ Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, Troy, NY 12180, USA
⁴ Kapteyn Astronomical Institute, P.O. Box 800, 9700 AV Groningen, The Netherlands
⁵ ISO Science Operations Centre, ESA Astrophysics Division, VILSPA, P.O. Box 50727, E-28080 Madrid, Spain
⁶ Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁷ NASA-Ames Research Center, MS 245-3, Moffett Field, CA 94035, USA
⁸ Space Research Organization Netherlands, P.O. Box 800, 9700 AV Groningen, The Netherlands

Received 19 August 1997 / Accepted 3 June 1998

Abstract

We present ISO-SWS spectroscopy of eight strong infrared sources with large extinction through the diffuse interstellar medium. These are five late-type Wolf-Rayet stars, the blue hypergiant Cyg OB2 #12 and the Galactic Center Sources 3 and 4. The spectra show a number of absorption features that can be ascribed to interstellar dust and gas. Features at 3.0, 3.4, 4.66 and 5.95 µm were already known from ground and airborne observations, while features at 4.27 and 6.21 µm are for the first time observed by ISO. We ascribe the 3.0, 4.27 and 4.66 µm features to ice, ice and gaseous CO, respectively, located in dense clouds along four lines-of-sight in our study. Based on an excellent correspondence with the well-known emission feature observed in many other objects, the 6.2 µm absorption band is identified with aromatic materials. The profile of the feature indicates that its carriers are PAHs or PAH clusters containing up to a few thousand carbon atoms. The lack of strong infrared features of hydroxy (OH), carbonyl (C=O) and aromatic CH groups towards the objects of the present study shows that the carbonaceous dust material in the diffuse ISM is poor in oxygen as well as hydrogen.

Key words: infrared: ISM: lines and bands Galaxy: abundances ISM: dust, extinction methods: observational

* Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.

Send offprint requests to: W.A. Schutte

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Contents

• 1. Introduction
• 2. Nature of the sources
• 3. Observations
• 4. The interstellar absorption features
  • 4.1. Comparison with earlier observations
  • 4.2. The 3.0, 4.27 and 4.66 µm features
  • 4.3. The 3.4 µm feature
  • 4.4. The 6.2 µm feature
  • 4.5. The 5.95 µm feature
• 5. Gaseous vs. solid state origin of the aromatic absorbing material
  • 5.1. Abundance constraints
  • 5.2. Spectroscopic constraints
  • 5.3. Discussion
• 6. Composition of the carbonaceous grain component
• 7. Discussion
• 8. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1998

Online publication: August 6, 1998
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