Abstract Details

Name: Gargi Shaw
Affiliation: Department of Astronomy and AStrophysics, Tata Instiute of Fundamental Research
Conference ID: ASI2021_170
Title : Roll of Polycyclic Aromatic Hydrocarbons on the Cosmic-Ray ionization rate in the Galaxy
Authors and Co-Authors : Author 1: Gargi Shaw, Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India // Author 2: Gary. J. Ferland, Department of Physics and Astronomy, University of Kentucky, Lexington, KY 40506, USA
Abstract Type : Oral
Abstract Category : Stars, ISM and 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.