Non-equilibrium chemistry and radiation hydrodynamics in metal-poor and metal-rich interstellar gas
Abstract
We present a flexible implementation of non-equilibrium chemical networks with the KROME framework, coupled to a nonlocal variable Eddington tensor-closed radiation transport scheme (VETTAM). Our fiducial network is designed to explicitly capture the formation pathways of important molecular species such as CO, OH, and H2O, intermediaries like H3+, 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 ≤ Z ≲ Z⊙) and densities (n ≲ 1017 cm−3). We confirm earlier works that suggest that OH and H2O are the key molecular species in dense gas (n ≳ 105 cm−3) for Z≲ 0.01 Z⊙, 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 ≳ 103 cm−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.
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BibTeX
@misc{menon2026nonequilibrium,
title = {{Non-equilibrium chemistry and radiation hydrodynamics in metal-poor and metal-rich interstellar gas}},
author = {Shyam H. Menon and Piyush Sharda and Blakesley Burkhart and Tommaso Grassi},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.17577},
eprint = {2610.17577},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.17577},
}
RIS
TY - UNPB
TI - Non-equilibrium chemistry and radiation hydrodynamics in metal-poor and metal-rich interstellar gas
AU - Shyam H. Menon
AU - Piyush Sharda
AU - Blakesley Burkhart
AU - Tommaso Grassi
PY - 2026
DA - 2026/10/13
PB - arXiv
DO - 10.48550/arxiv.2610.17577
UR - https://arxiv.org/abs/2610.17577
AB - We present a flexible implementation of non-equilibrium chemical networks with the KROME framework, coupled to a nonlocal variable Eddington tensor-closed radiation transport scheme (VETTAM). Our fiducial network is designed to explicitly capture the formation pathways of important molecular species such as CO, OH, and H_2O, intermediaries like 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 ≤ Z ≲ Z_⊙) and densities (n ≲ 10^17 cm^−3). We confirm earlier works that suggest that OH and H_2O are the key molecular species in dense gas (n ≳ 10^5 cm^−3) for Z≲ 0.01 Z_⊙, 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 ≳ 10^3 cm^−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.
ER -