a1 Sackler School of Physics & Astronomy, Tel Aviv University, Israël
a2 Harvard-Smithsonian Center for Astrophysics, Cambridge
a3 Department of Physics, Ohio State University, USA
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 ≲10-3times 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.
(Online publication November 25 2011)