The aromatic infrared bands as seen by ISO-SWS:: Probing the PAH model

We discuss the Aromatic Infrared Band (AIB) profiles observed by ISO-SWS towards a number of bright interstellar regions where dense molecular gas is illuminated by stellar radiation. Our sample spans a broad range of excitation conditions (exciting radiation fields with effective temperature, T-eff, ranging from 23 000 to 45 000 K). The SWS spectra are decomposed coherently in our sample into Lorentz profiles and a broadband continuum. We find that the individual profiles of the main AIBs at 3.3, 6.2, 8.6 and 11.3 mum are well represented with at most two Lorentzians. The 7.7 mum-AIB has a more complex shape and requires at least three Lorentz profiles. Furthermore, we show that the positions and widths of these AIBs are remarkably stable (within a few cm(-1)) confirming, at higher spectral resolution, the results from ISOCAM-CVF and ISOPHOT-S. This spectral decomposition with a small number of Lorentz profiles implicitly assumes that most of the observed bandwidth arises from a few, large carriers. Boulanger et al. (1998b) recently proposed that the AIBs are the intrinsic profiles of resonances in small carbon clusters. This interpretation can be tested by comparing the AIB profile parameters (band position and width) given in this work to laboratory data on relevant species when it becomes available. Taking advantage of our decomposition, we extract the profiles of individual AIBs from the data and compare them to a state-of-the-art model of Polycyclic Aromatic Hydrocarbon (PAH) cation emission. In this model, the position and width of the AIBs are rather explained by a redshift and a broadening of the PAH vibrational bands as the temperature of the molecule increases (Joblin et al. 1995). In this context, the present similarity of the AIB profiles requires that the PAH temperature distribution remains roughly the same whatever the radiation field hardness. Deriving the temperature distribution of interstellar PAHs, we show that its hot tail, which controls the AIB spectrum, sensitively depends on N-min (the number of C-atoms in the smallest PAH) and T-eff. Comparing the observed profiles of the individual AIBs to our model results, we can match all the AIB profiles (except the 8.6 mum-AIB profile) if N-min is increased with T-eff. This increase is naturally explained in a picture where small PAHs are more efficiently photodissociated in harsher radiation fields. The observed 8.6 mum-profile, both intensity and width, is not explained by our model. We then discuss our results in the broader context of ISO observations of fainter interstellar regions where PAHs are expected to be in neutral form.