2014/09/30 by Shmuel Bialy, Amiel Sternberg · 1 citation
Physics and Astronomy · #astro-ph.GA
paper · pdf · doi:10.1093/mnras/stv851
published as MNRAS, Volume 450, Issue 4, pp. 4424-4445 (2015) · Accepted for publication in MNRAS (with some improvements following referee report). 22 pages, 17 figures
arxiv created 2015/04/18 · arxiv updated 2015/09/09
We present numerical computations and analytic scaling relations for interstellar ion-molecule gas phase chemistry down to very low metallicities ( 10-3 × 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 H2, CO, CH, OH, H2O and O2, at gas temperatures ∼ 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, ζ/n, and metallicties, Z'. We find that the OH abundances are always maximal near the H-to-H2 conversion points, and that large OH abundances persist at very low metallicities even when the hydrogen is predominantly atomic. We study the OH/O2, C/CO and OH/CO abundance ratios, from large to small, as functions of ζ/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.