Hydrogenation and Charge States of Polycyclic Aromatic Hydrocarbons in Diffuse Clouds. II. Results

Abstract

We have modeled the states of hydrogenation and charge of polycyclic aromatic hydrocarbons (PAHs) in diffuse clouds for molecules ranging from benzene up to species containing 200 carbon atoms. It is found that the hydrogenation state of PAHs strongly depends on the size of the molecule. Small PAHs with fewer than about 15-20 carbon atoms are destroyed in most environments. Intermediate-size PAHs in the range of 20-30 carbon atoms are stripped of most of their peripheral hydrogen atoms, but may be able to survive in the interstellar medium because of the relative stability of their carbon skeleton upon UV photon absorption. Larger PAHs primarily have normal hydrogen coverage (i.e., with each peripheral carbon atom bearing a single hydrogen), with competition between this form and PAHs containing an additional hydrogen. Very large PAHs may be fully hydrogenated, with every peripheral carbon atom bearing two hydrogen atoms. Our finding that extremely dehydrogenated PAH neutrals or positively charged CmH, with m ranging from 15 to 30 and n ≤ 2, can survive in the interstellar medium contrasts with previous work, where it was generally assumed that PAHs losing their hydrogen coverage were quickly destroyed. A mechanism is proposed for the selective growth of these small dehydrogenated PAHs in diffuse clouds with respect to larger PAHs. Finally, our results are compared to previous studies on the hydrogenation and charge states of PAHs.