Determine the effective low-temperature rate coefficient for HCO+ destruction by atomic carbon

Determine the reaction mechanism and effective low-temperature rate coefficient for the destruction reaction C + HCO+ -> CO + CH+, including the possible role of intersystem crossing between electronic spin surfaces, to establish whether this pathway is suppressed or operative under astrophysical conditions.

Background

The extreme HCO+ abundances predicted with the UMIST22 network arise largely because the reaction C + HCO+ -> CO + CH+ is absent from that network, whereas it is included in UMIST12. Recent theoretical work indicates that the direct product channels may be strongly endoergic and possess substantial activation barriers, challenging the reaction’s assumed efficiency in cold gas.

The paper notes that the reaction could behave differently at high temperatures, such as those reached in fast shocks, and that intersystem crossing could modify the accessible reaction pathways. Consequently, the mechanism and effective low-temperature rate coefficient remain unresolved and require theoretical and experimental investigation.

References

It is also possible that the reaction mechanism is more complex than a simple barrier-mediated process. One possible source of this uncertainty is intersystem crossing between different electronic spin surfaces, which may alter the reaction pathways available to the system. Consequently, the effective low-temperature rate coefficient remains uncertain. Further theoretical and experimental studies are therefore needed to better constrain the reaction mechanism and the corresponding rate coefficient.

— The curious case of HCO$^+$: Extreme abundances under extreme conditions  (2608.21138 - Dutkowska et al., 21 Aug 2026) in Section 4, subsection “Chemical uncertainties and observational considerations”