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Astron. Astrophys. 339, 194-200 (1998)


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The shape of the unidentified infra-red bands: analytical fit to ISOCAM spectra

F. Boulanger 1, P. Boisssel 2, D. Cesarsky 1 and C. Ryter 3

1 Institut d'Astrophysique Spatiale, Université Paris Sud, Bat. 121, F-91405 Orsay Cedex, France
2 Laboratoire de Photophysique Moléculaire, Laboratoire du CNRS, Université Paris Sud, Bat. 213, F-91505 Orsay Cedex, France
3 DAPNIA/Service d'Astrophysique, CEA/Saclay, F-91191 Gif Sur Yvette Cedex, France

Received 27 February 1998 / Accepted 1 July 1998

Abstract

Modelling the unidentified infrared dust bands at 6.2 7.7, 8.6, 11.3, and 12.7 µm by Lorentz profiles is shown to produce excellent fits to the spectra obtained with the Continuous Variable Filter (CVF) of the instrument ISOCAM, on-board the Infrared Space Observatory (ISO). The feature pedestals are accounted for by the wide wings of the Lorentz curves which also account for a large fraction of the 10 µm "continuum". The Lorentz shape of the features is believed to reveal intrinsic properties of the emitters, i.e. a very short life time of the excited state, of the order of a few [FORMULA]. This is attributed to internal vibrational redistribution (IVR) between thermally accessible modes of the particles, which can be raised at high temperature by absorption of a single photon. Particles of a few hundred atoms may meet both requirements.

Key words: ISM: clouds – ISM: Rho-Ophiuchi cloud – ISM: NGC 7023 – infrared: ISM: lines and bands – infrared: line formation

Send offprint requests to: F. Boulanger

© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998

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