Role of Polycyclic Aromatic Hydrocarbons on the Cosmic-Ray ionization rate in the Galaxy

Abstract

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.

Accepted for publication in ApJ, 31 pages, 22 figures. A typo in the title is corrected

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