Role of Polycyclic Aromatic Hydrocarbons on the Cosmic-Ray Ionization Rate in the Galaxy
Abstract
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 ${{\rm{H}}}_{3}^{+}$ and H2. The presence of polycyclic aromatic hydrocarbons (PAHs) affects the free electron density, which changes the ${{\rm{H}}}_{3}^{+}$ 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 within a range of PAH abundances and show their effects on the ${{\rm{H}}}_{3}^{+}$ abundance. We predict an average CR ionization rate for H2 (ζ(H2)) = (7.88 ± 2.89) × 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) × 10-16 s-1 for models without PAHs.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- February 2021
- DOI:
- 10.3847/1538-4357/abdab2
- arXiv:
- arXiv:2101.03732
- Bibcode:
- 2021ApJ...908..138S
- Keywords:
-
- Astrochemistry;
- Interstellar molecules;
- Interstellar medium;
- Photodissociation regions;
- 75;
- 849;
- 847;
- 1223;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- Accepted for publication in ApJ, 31 pages, 22 figures. A typo in the title is corrected