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

Abstract

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, Teff, 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 microns are well represented with at most two lorentzians. Furthermore, we show that the positions and widths of these AIBs are remarkably stable (within a few cm-1). We then extract the profiles of individual AIBs from the data and compare them to a model of Polycyclic Aromatic Hydrocarbon (PAH) cation emission which includes the temperature dependence of the AIB profiles. 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 Nmin (the number of C-atoms in the smallest PAH) and Teff. Comparing the observed profiles of the individual AIBs to our model results, we can match most of the AIB profiles if Nmin is increased with Teff. 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.