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

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1. Introduction

The unidentified infrared (UIR) dust bands at 3.3, 6.2, 7.7, 8.6, 11.3, and 12.7 µm, observed in different kind of celestial nebulae, have been widely attributed to aromatic C-C and C-H bindings in molecules and/or larger particles. Combinations of Polycyclic Aromatic Hydrocarbons molecules (PAHs) have been proposed to account for the observed interstellar features, based on laboratory experiments and quantum mechanical calculations on molecules with sizes up to a few tens of atoms (Léger & Puget 1984, for reviews see Allamandola et al. 1987, Puget & Léger 1989). A good spectral match between the interstellar bands and terrestrial coals with a high degree of aromatic substructures have been demonstrated. In regions of intense radiation fields, such as in proto-planetary nebulae, large solid particles can be hot enough to account for the bands (Guillois et al. 1994).

Thanks to its high sensitivity, the Infrared Space Observatory ISO (Kessler et al. 1996) has enormously added to the knowledge in this field. First, the bands which were implicitly associated with nebulae have been shown to be present in cirrus spectra, where the radiation field is so low that there is no alternative mechanism to the transient heating of small particles by absorption of a single photon (Mattila et al. 1996; Boulanger et al. 1998). Models of emission from a continuous size distribution of particles indicate that the mid-infrared radiation of the ISM is dominated by particles from one hundred to one thousand atoms (Désert et al. 1990, Schutte et al. 1993). Second, spectra obtained at high resolution with the Short Wavelength Spectrometer (SWS, Verstraete et al. 1996, Roelfsema et al. 1996) indicate that the band profiles are smooth with only little suggestion of resolved substructures. Finally, the spectra show emission between the main dust features (Boulanger et al. 1996).

For this work, we have analysed spectra obtained on galactic objects, taken from a program aimed at selecting, as much as possible, simple and uniquely specified physical situations and low extinction. They have been obtained with the rotating Circular Variable Filter (CVF) of the camera ISOCAM (Cesarsky et al. 1996a) on-board the satellite ISO. We discover that the UIR dust bands as displayed by ISOCAM are best represented by Lorentz profiles with wide wings, contrasting with the Gauss profiles which have narrower wings. This paper presents this decomposition of the spectra and discusses its physical signification.

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© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998