On the Inference of the Cosmic Ray Ionization Rate $\zeta$ from the HCO$^+$-to-DCO$^+$ Abundance Ratio: The Effect of Nuclear Spin

TL;DR

This study reexamines the use of the [HCO$^+$]/[DCO$^+$] ratio as a probe for cosmic ray ionization rates in dense interstellar regions, emphasizing the impact of nuclear spin states and initial conditions on its reliability.

Contribution

Introduces a new chemical network accounting for nuclear spin states, enhancing the understanding of the [HCO$^+$]/[DCO$^+$] ratio's dependence on various physical parameters.

Findings

The ratio's sensitivity to cosmic ray ionization rate increases with source age.

Initial ortho-to-para ratio significantly affects the ratio's diagnostic utility.

The method aligns with observations near supernova remnants and star-forming cores.

Abstract

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.