Photodestruction of PAHs in the interstellar medium --- II. Influence of the states of ionization and hydrogenation

T. Allain, S. Leach, and E. Sedlmayr

Astronomy & Astrophysics, Vol. 305, 616-630 (1996)

Gzipped PostScript version (297KB)

Abstract:

We have extended the model for interstellar polycyclic aromatic hydrocarbons (PAHs) of Allain et al. (1995) in order to describe ionized and partially dehydrogenated PAHs and hence provide a more realistic description of these species in the interstellar medium (ISM). When ionized and partially dehydrogenated, PAHs are less stable against photodestruction than their parent molecules. For instance, an ovalene cation with 32 carbon atoms has photodissociation rates 4 times greater than the corresponding molecule, whereas the corresponding totally dehydrogenated molecule has photodissociation rates 3 orders of magnitude greater. However, in the regions where the unidentified infrared (UIR) emission bands have been observed (reflection nebulae, planetary nebulae, HII regions, active galactic nuclei...) only PAHs with less than 50 carbon atoms may be considerably dehydrogenated. In contrast, the fraction of ionized PAHs is quite similar for all molecular sizes of PAHs but its
value largely depends on the physical properties of the considered medium. A low ionization fraction is always found to be associated with relatively high hydrogen coverage of small PAHs. After taking into account these ionization and hydrogenation states we conclude that PAHs with more than 50 carbon atoms may survive the UV radiation field in the regions of the ISM where the UIR bands are observed. The smaller PAHs are all destroyed within some years in the media of high UV field which exhibit the UIR bands.

Keywords:

ISM: molecules - molecular processes - ultraviolet: ISM