Astronomy & Astrophysics Seminar: Molecules at the Era of First Metal Enrichment

Abstract:

I will discuss cold dense-gas interstellar chemistry down to very low metallicities (1e-3 solar), and/or up to high driving ionization rates, appropriate for the cool ISM in low-metallicity dwarf galaxies and especially for early enriched clouds during the reionization and Pop-II star formation era.  I will show computations for partially shielded gas for steady-state conditions, for a wide range of ionization parameters zeta/n, and metallicities Z', focusing on the
behavior of H_2, CO, CH, OH, H_2O and O_2.

For solar metallicity shielded gas clouds in the Galaxy, CO is the most abundant heavy molecule, and CO is widely used to study the physical conditions in these clouds and also to infer H_2 masses via the CO-to-H_2 conversion factor. We find that with decreasing Z' (and/or increasing zeta/n) CO vanishes, and a transition from "CO-dominated gas" to "OH-dominated gas" occurs. Thus, OH may be a better molecular tracer of the low metallicity clouds at the era of first metal enrichment.

I will also discuss our recent finding that water molecules may have been abundant in low metallicity gas clouds, during the epoch of first metal enrichment of the ISM.  The high temperatures (few hundreds K) expected in low metallicity-gas environments compensate for the low oxygen level and the ineffective dust shielding, and allow the formation of H_2O molecules, with abundances comparable to those found in the diffuse ISM in our Galaxy today.

Seminar Organizer: Prof. Rennan Barkana