The Transition from Population III to Population II Stars

The transition from Population III to Population II stars is determined bythe presence of a sufficient amount of metals, in particular, oxygen andcarbon. The vastly different yields of these relevant metals between differentinitial stellar mass functions would then cause such a transition to occur atdifferent times. We show that the transition from Pop III to Pop II stars islikely to occur before the universe can be reionized, if the IMF is entirelyvery massive stars (M > 140 M_sun). A factor of about 10 more ionizing photonswould be produced in the case with normal top-heavy IMF (e.g., M ~ 10-100M_sun), when such a transition occurs. Thus, a high Thomson optical depth(tau_e >= 0.11-0.14) may be indication that the Population III stars possess amore conventional top-heavy IMF.Comment: 4 pages, 2 figures. Minor revisions, accepted by ApJ Letter