Unidentified infrared emission bands in the diffuse interstellar medium

Using the mid-infrared spectrometer on board the Infrared Telescope in Spac e and the low-resolution grating spectrometer (PHT-S) on board the Infrared Space Observatory, we obtained 820 mid-infrared (5-12 mum) spectra of the diffuse interstellar medium (DIM) in the Galactic center, W51, and Carina N ebula regions. These spectra indicate that the emission is dominated by the unidentified infrared (UIR) emission bands at 6.2, 7.7, 8.6, and 11.2 mum. The relative band intensities (6.2/7.7 mum, 8.6/7.7 mum, and 11.2/7.7 mum) were derived from these spectra, and no systematic variation in these rati os was found in our observed regions, in spite of the fact that the inciden t radiation intensity differs by a factor of 1500. Comparing our results wi th the polycyclic aromatic hydrocarbons (PAHs) model for the UIR band carri ers, we find that PAHs in the DIM have no systematic variation in their siz e distribution, their degree of dehydrogenation is independent of the stren gth of UV radiation field, and they are mostly ionized. The finding that PA Hs in the DIM with low UV radiation field strength are mostly ionized is in compatible with past theoretical studies, in which a large fraction of neut ral PAHs is predicted in this kind of environment. A plausible resolution o f this discrepancy is that the recombination coefficients for electron and large PAH positive ion are by at least an order of magnitude less than thos e adopted in past theoretical studies. Because of the very low population o f neutral state molecules, photoelectric emission from interstellar PAHs is probably not the dominant source of heating of the diffuse interstellar ga s. The present results imply constant physical and chemical properties of t he carriers of the UIR emission bands in the DIM covering the central and d isk regions of the Galaxy, which could help in the identification of the ca rriers.