The diffuse interstellar bands at 5797, 6379 and 6613 Å: Ionization properties of the carriers

We present a study of the behaviour and ionization properties of three narrow Diffuse Interstellar Bands (DIBs) at λλ5797, 6379 and 6613 Å. In all three DIBs substructures have recently been detected, indicating large gaseous molecular carriers. Studying DIBs in regions with drastically different physical properties in terms of UV flux and density enables us to monitor the behaviour of the carriers and hence to constrain their nature. We observed these three DIBs along 40 different lines-of-sight (35 program stars and 5 standard stars) consisting of HII regions, dark clouds, molecular clouds and reflection nebulae. The DIB variations at low reddening are explained by a new model of photoionization equilibrium of the DIB carriers. This model takes into account the penetration depth of UV ionizing photons throughout the cloud. The slope of the variation of DIB strength as a function of reddening thus allows us to estimate the effective ionization potentials of the carriers. Following this new analysis, the carriers of the λ5797 and λ6613 Å DIBs would have ionization potentials above 10 eV, reminiscent of large PAHs or fullerenes which have a single positive charge. The estimated ionization potential (7-9 eV) of the λ6379 Å DIB seems to indicate a large neutral carrier.