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

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.

arXiv commentApJ (accepted), 21 pages, 7 figures

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Cited by in corpus (14)

• On Cosmic Ray-Driven Grain Chemistry in Cold Core Models
• Isomers in Interstellar Environments (I): The Case of Z- and E-Cyanomethanimine
• Magnetically regulated collapse in the B335 protostar? II. Observational constraints on gas ionization and magnetic field coupling
• A new proxy to estimate the cosmic-ray ionisation rate in dense cores
• Tracers of the ionization fraction in dense and translucent gas: I. Automated exploitation of massive astrochemical model grids
• Cold Clouds as Cosmic-Ray Detectors
• Dependence of Chemical Abundance on the Cosmic Ray Ionization Rate in IC 348
• A new analytic approach to infer the cosmic-ray ionization rate in hot molecular cores from HCO${}^{+}$, N${}_2$H${}^{+}$, and CO observations
• C-type shock modelling -- the effect of new H${}_2$-H collisional rate coefficients
• Modelling cosmic masers in C-type shock waves -- the coexistence of Class I CH3OH and 1720 MHz OH masers
• Impact of cosmic-ray propagation on the chemistry and ionisation fraction of dark clouds
• Collisional Pumping of OH Masers near Supernova Remnants
• Collisional Pumping of H${}_2$O and CH${}_3$OH Masers in C-Type Shock Waves
• Chemical evolution of cold dark clouds in the vicinity of supernova remnants