The evolution of HCO$^{+}$ in molecular clouds using a novel chemical post-processing algorithm

Abstract: Modeling the chemistry of molecular clouds is critical to accurately simulating their evolution. To reduce computational cost, 3D simulations generally restrict their chemistry to species with strong heating and cooling effects. Time-dependent information about the evolution of other species is therefore often neglected. We address this gap by post-processing tracer particles in the SILCC-Zoom molecular cloud simulations. Using a chemical network of 39 species and 301 reactions (including freeze-out of CO and H${2}$O), and a novel algorithm to reconstruct a density grid from sparse tracer particle data, we produce time-dependent density distributions for various species. We focus upon the evolution of HCO$^{+}$, which is a critical formation reactant of CO but is not typically modeled on-the-fly. We find that $\sim90$% of the HCO$^{+}$ content of the cold molecular gas forms in situ around $n\textrm{{HCO$^+$}}\simeq10^3$-$10^4$ cm$^{-3}$, over a time-scale of approximately 1 Myr. The remaining $\sim10$% forms at high extinction sites, with minimal turbulent mixing out into the less dense gas. We further show that the dominant HCO$^{+}$ formation pathway is dependent on the visual extinction, with the reaction H${3}^{+}$ + CO contributing 90% of the total HCO$^{+}$ production above $A\textrm{V,3D}=3$. We produce the very first maps of the HCO$^{+}$ column density, $N$(HCO$^{+}$), and show that it reaches values as high as $10^{15}$ cm$^{-2}$. We find that 50% of the HCO$^+$ mass is located within $A_\textrm{V}\sim10$-30, in a density range of $10^{3.5}$-$10^{4.5}$ cm$^{-3}$. Our maps of $N$(HCO$^{+}$) are shown to be in good agreement with recent observations of the W49A star-forming region.