Charge Variation of Polycyclic Aromatic Hydrocarbons in the Diffuse Interstellar Medium

Abstract

We have modelled the charge balance for several polycyclic aromatic hydrocarbon (PAH) molecules, the fullerene C₆₀ and small graphitic grains in the diffuse interstellar medium (ISM). For the smaller PAH molecules, we used pyrene (C₁₆H₁₀), pentacene (C₂₂H₁₄) and coronene (C₂₄H₁₂). We then calculated the charge distribution for each particle as a function of electron density, radiation field ana depth at which it was situated in a diffuse cloud. For typical conditions in the diffuse ISM, the charge distribution is determined to a large extent by PAH electron affinity, which determines whether an electron will stick to it. For the PAH molecules, we compare the charge distribution calculated using experimentally measured cation-electron recombination rates with theoretically determined rates. We conclude that while a PAH mixture may exist in a variety of singley charged and neutral states, larger disk shaped and spherical grains will be predominantly neutral.