1 Introduction

The family of infrared features at 3.3, 6.2, 7.7, 8.6, 11.3 and 12.7 $\mu$m has been observed towards a large number of sightlines in the Galaxy and in other galaxies since the nineteen seventies. Early on, it was recognized that these bands correspond to vibrational modes in carbonaceous aromatic systems (Duley & Williams 1981; Léger & Puget 1984; Allamandola et al. 1985). These dust bands are therefore dubbed the Aromatic Infrared Bands (hereafter AIBs). The bands at 3.3, 8.6, 11.3 and 12.7 $\mu$m stem from vibrational modes of the aromatic C-H bond; the remaining bands are ascribed to vibrations of the aromatic C-C bonds. The Infrared Space Observatory (ISO, Kessler et al. 1996) mission has provided us with an unprecedented wealth of data in this context.

Comparative studies of the AIBs in a wide variety of environments have been made possible by the high sensitivity of the ISO camera (ISOCAM, Cesarsky C. et al. 1996) with its circular variable filter and of the ISOPHOT-S spectrophotometer (Lemke et al. 1996), which both have low spectral resolutions ( $\lambda / \Delta\lambda \sim 40$ and 90 respectively: Boulanger et al. 1996, 1998a, 1998b; Cesarsky et al. 1996a, 1996b, 2000a, 2000b; Crété et al. 1999; Klein et al. 1999; Laureijs et al. 1996; Mattila et al. 1996; Persi et al. 1999; Uchida et al. 1998, 2000). The AIB profiles as seen in these data are very similar (in position and width) over a range of objects where the stellar radiation field and effective temperature vary greatly (1 to 10⁴ times the standard interstellar radiation field, $T_{\rm eff}=11$000 to 50000 K). The profile invariance as well as the large width of the AIBs lead Boulanger et al. (1998b) to conclude that the carriers of these bands are large aromatic systems containing more than 50 C-atoms.

The Short Wavelength Spectrometer (SWS, de Graauw et al. 1996) onboard ISO, less sensitive than ISOCAM but with higher spectral resolution ( $\lambda/\Delta\lambda=1000$) and broader wavelength coverage (2.4-45 $\mu$m), brings a better view of the interstellar AIBs in the brightest regions. Such detailed data have the ability to constrain the nature and physical state of the band carriers (Beintema et al. 1996; Molster et al. 1996; Roelfsema et al. 1996; Verstraete et al. 1996; Moutou et al. 2000; van Kerckhoven et al. 2000; Hony et al. 2000). In the first part of this paper, we decompose the AIB spectrum into Lorentz profiles and a broadband continuum in order to characterize (band position and width) each individual AIB and to compare them between objects. In the second part of the paper, we compare these new observations with a model considering free-flying aromatic molecules (Polycyclic Aromatic Hydrocarbons or PAHs) as the origin of the AIBs. Indeed, the presence of AIBs in the low-excitation diffuse interstellar medium (Boulanger et al. 1996; Mattila et al. 1996; Onaka et al. 1996) requires the existence of free-flying PAHs or small grains excited by starlight; furthermore, these PAHs or grains must be small enough to undergo strong temperature fluctuations leading to emission of the AIBs in the near-infrared (Sellgren 1984). In this emission mechanism, the shape of the emergent AIB spectrum only depends on the radiation field hardness (or $T_{\rm eff}$, see Sect. 2) and not on the flux of stellar photons (parameterized as $\chi$ in Sect. 2).

In Sect. 2, we present the SWS spectra. The observed AIB profiles are characterized in Sect. 3. These results are compared to the predictions of the PAH model in Sect. 4. We summarize and discuss the significance of our results in Sect. 5.