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2017ApJ...845..163N - Astrophys. J., 845, 163-163 (2017/August-3)

The cosmic-ray ionization rate in the galactic disk, as determined from observations of molecular ions.


Abstract (from CDS):

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+, H2O+, and H3+. For diffuse atomic clouds at Galactocentric distances in the range Rg∼4–9 kpc, observations of ArH+, OH+, and H2O+ imply a mean primary CRIR of (2.2±0.3) exp[(R0-Rg)/4.7 kpc]×10–16 s–1 per hydrogen atom, where R0=8.5 kpc. Within diffuse molecular clouds observed toward stars in the solar neighborhood, measurements of H3+ 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, AV(tot), with a best-fit dependence ∝AV(tot)–1 for AV(tot) 0.5.

Abstract Copyright: © 2017. The American Astronomical Society. All rights reserved.

Journal keyword(s): astrochemistry - cosmic rays - ISM: clouds - ISM: molecules - molecular processes - submillimeter: ISM - submillimeter: ISM

Simbad objects: 33

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