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

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, $Z^{\prime }$. 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 $Z^{\prime }$. Much of the cold dense ISM for the Pop-II generation may have been OH-dominated and atomic rather than CO-dominated and molecular.

Accepted for publication in MNRAS (with some improvements following referee report). 22 pages, 17 figures

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