On the Inference of the Cosmic Ray Ionization Rate $\zeta$ from the
  HCO$^+$-to-DCO$^+$ Abundance Ratio: The Effect of Nuclear Spin
  The chemistry of dense interstellar regions was analyzed using a
time-dependent gas-grain astrochemical simulation and a new chemical network
that incorporates deuterated chemistry taking into account nuclear spin-states
for the hydrogen chemistry and its deuterated isotopologues. With this new
network, the utility of the [HCO$^+$]/[DCO$^+$] abundance ratio as a probe of
the cosmic ray ionization rate has been reexamined, with special attention paid
to the effect of the initial value of the molecular hydrogen ortho-to-para
ratio (OPR). After discussing the use of the probe for cold cores, we then
compare our results with previous theoretical and observational results for a
molecular cloud close to the supernova remnant W51C, which is thought to have
an enhanced cosmic ray ionization rate $\zeta$ caused by the nearby
$\gamma$-ray source. In addition, we attempt to use our approach to estimate
the cosmic ray ionization rate for L1174, a dense core with an embedded star.
Beyond the previously known sensitivity of [HCO$^+$]/[DCO$^+$] to $\zeta$, we
demonstrate its additional dependence on the initial OPR and, secondarily, on
the age of the source, its temperature, and its density. We conclude that the
usefulness of the [HCO$^+$]/[DCO$^+$] abundance ratio to constrain the cosmic
ray ionization rate in dense regions increases with source age and ionization
rate as the ratio becomes far less sensitive to the initial value of the OPR.
