CO/H2, C/CO, OH/CO, and OH/O2 in Dense Interstellar Gas: From High Ionization to Low Metallicity

TL;DR

This paper presents numerical and analytical models of interstellar ion-molecule chemistry at very low metallicities and high ionization rates, revealing persistent OH abundance and atomic dominance in early or low-metallicity environments.

Contribution

It provides new scaling relations and insights into the chemical composition of low-metallicity, high-ionization interstellar gas, extending understanding to environments like dwarf galaxies and early universe clouds.

Findings

OH abundance peaks near H-to-H$_2$ conversion points

Large OH persists at low metallicities with atomic hydrogen

Cold dense ISM in early environments may be OH-dominated

Abstract

We present numerical computations and analytic scaling relations for interstellar ion-molecule gas phase chemistry down to very low metallicities ($ 10^{-3} \times$ solar), and/or up to high driving ionization rates. Relevant environments include the cool interstellar medium (ISM) in low-metallicity dwarf galaxies, early enriched clouds at the reionization and Pop-II star formation era, and in dense cold gas exposed to intense X-ray or cosmic-ray sources. We focus on the behavior for H$_2$, CO, CH, OH, H$_2$O and O$_2$, at gas temperatures $\sim 100$ K, characteristic of a cooled ISM at low metallicities. We consider shielded or partially shielded one-zone gas parcels, and solve the gas phase chemical rate equations for the steady-state "metal-molecule" abundances for a wide range of ionization parameters, $\zeta/n$, and metallicties, $Z'$. We find that the OH abundances are always maximal near the H-to-H$_2$ conversion points, and that large OH abundances persist at very low metallicities even when the hydrogen is predominantly atomic. We study the OH/O$_2$, C/CO and OH/CO abundance ratios, from large to small, as functions of $\zeta/n$ and $Z'$. Much of the cold dense ISM for the Pop-II generation may have been OH-dominated and atomic rather than CO-dominated and molecular.