Non-equilibrium chemistry and radiation hydrodynamics in metal-poor and metal-rich interstellar gas

We present a flexible implementation of non-equilibrium chemical networks with the $\mathtt{\text{KROME}}$ framework, coupled to a nonlocal variable Eddington tensor-closed radiation transport scheme ($\mathtt{\text{VETTAM}}$). Our fiducial network is designed to explicitly capture the formation pathways of important molecular species such as CO, OH, and ${\mathrm{H}}_2\mathrm{O}$, intermediaries like ${\mathrm{H}}_3^{+}$, and the destruction of these species through photon and cosmic-ray dissociation. We validate our scheme through benchmark tests, and demonstrate its success in 1D and 3D problems covering a broad range of metallicities ($0\le Z\lesssim {\mathrm{Z}}_{\odot }$) and densities ($n\lesssim {10}^{17}\mathrm{c}{\mathrm{m}}^{-3}$). We confirm earlier works that suggest that OH and ${\mathrm{H}}_2\mathrm{O}$ are the key molecular species in dense gas ($n\gtrsim {10}^5\mathrm{c}{\mathrm{m}}^{-3}$) for $Z\lesssim 0.01{\mathrm{Z}}_{\odot }$, dominate the cooling at these densities, and demonstrate that our scheme can reproduce their abundances with similar accuracy as dedicated static photo-dissociation region (PDR) codes. We also find that the dominant fine-structure coolant in dense ($n\gtrsim {10}^3\mathrm{c}{\mathrm{m}}^{-3}$) PDRs transitions from CI to CII to OI at increasingly lower $Z$. Finally, we find that in turbulent media, rapid mixing at PDR interfaces renders the chemical species out of equilibrium, and show that this could potentially alleviate the long-standing problem of (static) PDR models producing excess CI in the diffuse ISM over UV absorption-line measurements. We conclude by presenting and comparing with alternative implementations of reduced but faster chemical networks in our scheme, discussing the problem-dependent performance-accuracy tradeoffs and future prospects.