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# cosmic ray background 2017

Reported by: Owned by: Gary J. Ferland nobody good to do C19_branch chemical network trunk

### Description

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Title:
The cosmic ray ionization rate in the Galactic disk, as determined from observations of molecular ions
Authors:
Neufeld, David A.; Wolfire, Mark G.
Publication:
eprint arXiv:1704.03877
Publication Date:
04/2017
Origin:
ARXIV
Keywords:
Astrophysics - Astrophysics of Galaxies
Comment:
48 pages, including 3 tables (at end) and 13 figures. Accepted for publication in the Astrophysical Journal
Bibliographic Code:
2017arXiv170403877N
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
$R_g \sim 4 - 9$ kpc, observations of ArH$^+$, OH$^+$, and H$_2$O$^+$
imply a mean primary CRIR of $(2.2 \pm 0.3) \exp [(R_0-R_g)/4.7\,\ m{kpc}] \times 10^{-16} \ m \, s^{-1}$ 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 H$_2$ imply a primary CRIR of
$(2.3 \pm 0.6) \times 10^{-16}\,\,\ m s^{-1}$ per H atom, corresponding to a
total ionization rate per H$_2$ molecule of $(5.3 \pm 1.1) \times 10^{-16}\,\,\ m s^{-1},$ 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 $2.9 \times 10^{-16}\,\,\ m s^{-1}$ per H atom. Our
results show marginal evidence that the CRIR in diffuse molecular clouds
decreases with cloud extinction, $A_{\ m V}({\ m tot})$, with a best-fit
dependence $\propto A_{\ m V}({\ m tot})^{-1}$ for $A_{\ m V}({\ m tot}) \ge 0.5$.


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