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#387 new enhancement

cosmic ray background 2017

Reported by: Gary J. Ferland Owned by: nobody
Priority: good to do Milestone: C19_branch
Component: chemical network Version: trunk
Keywords: Cc:


about 50% larger than our default

The cosmic ray ionization rate in the Galactic disk, as determined from observations of molecular ions
Neufeld, David A.; Wolfire, Mark G.
eprint arXiv:1704.03877
Publication Date:	
Astrophysics - Astrophysics of Galaxies
48 pages, including 3 tables (at end) and 13 figures. Accepted for publication in the Astrophysical Journal
Bibliographic Code:	

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 $(2.2 \pm 0.3) \exp [(R_0-R_g)/4.7\,\
\times 10^{-16} \
m \, s^{-1}$ 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 $(5.3 \pm 1.1) \times 
m s^{-1},$ 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 

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