Role of Polycyclic Aromatic Hydrocarbons on the Cosmic-Ray Ionization Rate in the Galaxy

Abstract

The cosmic-ray (CR) ionization rate (ζ, s−1) plays an important role in the interstellar medium. It controls ion–molecular chemistry and provides a source of heating. Here we perform a grid of calculations using the spectral synthesis code CLOUDY along nine sightlines toward, HD 169454, HD 110432, HD 204827, λ Cep, X Per, HD 73882, HD 154368, Cyg OB2 5, and Cyg OB2 12. The value of ζ is determined by matching the observed column densities of H 3 + and H2. The presence of polycyclic aromatic hydrocarbons (PAHs) affects the free electron density, which changes the H 3 + density and the derived ionization rate. PAHs are ubiquitous in the Galaxy, but there are also regions where PAHs do not exist. Hence, we consider clouds within a range of PAH abundances and show their effects on the H 3 + abundance. We predict an average CR ionization rate for H2 (ζ(H2)) = (7.88 ± 2.89) × 10−16 s−1 for models with average Galactic PAH abundances, (PAH/H = 10−6.52), except Cyg OB2 5 and Cyg OB2 12. The value of ζ is nearly 1 dex smaller for sightlines toward Cyg OB2 12. We estimate the average value of ζ(H2) = (95.69 ± 46.56) × 10−16 s−1 for models without PAHs.