Role of Polycyclic Aromatic Hydrocarbons on the Cosmic-Ray ionization
  rate in the Galaxy
  The cosmic-ray ionization rate ($\zeta$, 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 towards, HD 169454, HD 110432, HD
204827, $\lambda$ Cep, X Per, HD 73882, HD 154368, Cyg OB2 5, Cyg OB2 12. The
value of $\zeta$ is determined by matching the observed column densities of
H$_3^+$ and H$_2$. 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 with a range of PAH
abundances and show their effects on the H$_3^+$ abundance. We predict an
average cosmic-ray ionization rate for H$_2$ ($\zeta$(H$_2$))= (7.88 $\pm$
2.89) $\times$ 10$^{-16}$ s$^{-1}$ for models with average Galactic PAHs
abundances, (PAH/H =10$^{-6.52}$), except Cyg OB2 5 and Cyg OB2 12. The value
of $\zeta$ is nearly 1 dex smaller for sightlines toward Cyg OB2 12. We
estimate the average value of $\zeta$(H$_2$)= (95.69 $\pm$ 46.56) $\times$
10$^{-16}$ s$^{-1}$ for models without PAHs.
