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Astron. Astrophys. 363, L5-L8 (2000)

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Letter to the Editor

Dehydrogenation of polycyclic aromatic hydrocarbons in the diffuse interstellar medium

M.H.Vuong and B.H. Foing

Solar System Division, ESA Space Science Department, ESTEC/SO, 2200 AG Noordwijk, The Netherlands (vuong@obs-nice.fr, bfoing@estec.esa.nl)

Received 28 July 2000 / Accepted 17 October 2000


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).

Key words: ISM: molecules – ISM: clouds – ISM: dust, extinction

Present address: Observatoire de la Côte d'Azur, Département Fresnel, 06304 Nice Cedex, France

Send offprint requests to: Bernard H. Foing

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