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

Understand

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, $Z'$.