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

TL;DR

This study investigates how polycyclic aromatic hydrocarbons (PAHs) influence the cosmic-ray ionization rate in the Galaxy by modeling sightlines and comparing with observed molecular data, revealing PAHs significantly affect ionization estimates.

Contribution

It provides the first detailed modeling of PAH effects on cosmic-ray ionization rates across multiple sightlines using the CLOUDY code.

Findings

Average cosmic-ray ionization rate with PAHs: (7.88 ± 2.89) × 10^{-16} s^{-1}

Without PAHs, the ionization rate is approximately 10 times higher.

PAH presence reduces the estimated ionization rate by nearly 1 dex.

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.