2021ApJ...908..138S

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 H₃⁺ and H₂. The presence of polycyclic aromatic hydrocarbons (PAHs) affects the free electron density, which changes the H₃⁺ 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 H₃⁺ abundance. We predict an average CR ionization rate for H₂ (ζ(H₂)) = (7.88 ± 2.89) x 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) x 10^–16 s^–1 for models without PAHs.