Dehydrogenation of polycyclic aromatic hydrocarbons in the diffuse interstellar medium

We present a model for the hydrogenation states of Polycyclic Aromatic Hydrocarbons (PAHs) in the diffuse interstellar medium. First, we study the abundance of hydrogenation and charge states of PAHs due to photo-ionization, photo-dissociation in the interstellar UV field, electron recombination and chemical reactions between PAH cations and H or H₂. For PAH cations, we find that the dehydrogenation effects are dominant. The hydrogenation state of PAHs depends strongly on the H density, the size of molecule and UV field. In diffuse clouds with low H density and normal UV radiation, PAHs containing less than 40 C are completely or strongly dehydrogenated whereas at high H density, they are normally hydrogenated. The partially dehydrogenated species dominate in intermediate density clouds. PAHs above 40 C are quite stable and are fully hydrogenated, which would favor their spectroscopic search in near IR surveys of Diffuse Interstellar Bands (DIBs).