Molecular Clouds at the Reionization Epoch

Abstract

We show that for ionization-driven chemistry in molecular clouds with very sub-solar (but non-vanishing) metallicities, OH replaces CO as the most abundant molecule containing at least one heavy element. Old Population-II stars, with observed metallicities ≲10 -3 times solar, may have formed in such OH-dominated star-forming clouds, shortly after the heavy elements were first produced at the epoch of reionization in the early Universe.