Publication Date:
2015-05-28
Description:
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 behaviour for H 2 , CO, CH, OH, H 2 O and O 2 , 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, / 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 / 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.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
