The Cosmic-Ray Ionization Rate in the Galactic Disk, as Determined from Observations of Molecular Ions
Abstract
We have obtained estimates for the cosmic-ray ionization rate (CRIR) in the Galactic disk, using a detailed model for the physics and chemistry of diffuse interstellar gas clouds to interpret previously published measurements of the abundance of four molecular ions: ArH+, OH+, {{{H}}}2{{{O}}}+, and {{{H}}}3+. For diffuse atomic clouds at Galactocentric distances in the range {R}g∼ 4{--}9 {kpc}, observations of ArH+, OH+, and {{{H}}}2{{{O}}}+ imply a mean primary CRIR of (2.2+/- 0.3) \exp [({R}0-{R}g)/4.7 {kpc}]× {10}-16 {{{s}}}-1 per hydrogen atom, where {R}0=8.5 {kpc}. Within diffuse molecular clouds observed toward stars in the solar neighborhood, measurements of {{{H}}}3+ and H2 imply a primary CRIR of (2.3+/- 0.6)× {10}-16 {{{s}}}-1 per H atom, corresponding to a total ionization rate per H2 molecule of (5.3+/- 1.1)× {10}-16 {{{s}}}-1, in good accord with previous estimates. These estimates are also in good agreement with a rederivation, presented here, of the CRIR implied by recent observations of carbon and hydrogen radio recombination lines along the sight line to Cas A. Here, our best-fit estimate for the primary CRIR is 2.9× {10}-16 {{{s}}}-1 per H atom. Our results show marginal evidence that the CRIR in diffuse molecular clouds decreases with cloud extinction, {A}{{V}}({tot}), with a best-fit dependence \propto {A}{{V}}{({tot})}-1 for {A}{{V}}({tot})≥slant 0.5.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- August 2017
- DOI:
- 10.3847/1538-4357/aa6d68
- arXiv:
- arXiv:1704.03877
- Bibcode:
- 2017ApJ...845..163N
- Keywords:
-
- astrochemistry;
- cosmic rays;
- ISM: clouds;
- ISM: molecules;
- molecular processes;
- submillimeter: ISM;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 48 pages, including 3 tables (at end) and 13 figures. Accepted for publication in the Astrophysical Journal