Abundances in metal-poor stars and chemical evolution of the early Galaxy

We have attributed the elements from Sr through Ag in stars of low metallicities ([Fe/H] ≤ -1.5) to charged-particle reactions (CPR) in neutrino-driven winds, which are associated with neutron star formation in low-mass and normal supernovae (SNe) from progenitors of ∼ 8-11M _⊙ and ∼ 12-25M_⊙, respectively. Using this rule and attributing all Fe production to normal SNe, we previously developed a phenomenological two-component model, which predicts that [Sr/Fe] ≥ -0.32 for all metal-poor stars. This is in direct conflict with the high-resolution data now available, which show that there is a great shortfall of Sr relative to Fe in many stars with [Fe/H] ≤ -3. The same conflict also exists for the CPR elements Y and Zr. We show that the data require a stellar source leaving behind black holes and that hypernovae (HNe) from progenitors of ∼ 25-50M _⊙ are the most plausible candidates. If we expand our previous model to include three components (low-mass and normal SNe and HNe), we find that essentially all of the data are very well described by the new model. The HN yield pattern for the low - A elements from Na through Zn (including Fe) is inferred from the stars deficient in Sr, Y, and Zr. We estimate that HNe contributed ∼ 24% of the bulk solar Fe inventory while normal SNe contributed only ∼ 9% (not the usually assumed ∼ 33%). This implies a greatly reduced role of normal SNe in the chemical evolution of the low - A elements. This work was supported in part by US DOE grants DE-FG03-88ER13851 (G.J.W) and DE-FG02-87ER40328 (Y.Z.Q.). G.J.W acknowledges NASA's Cosmochemistry Program for research support provided through J. Nuth at the Goddard Space Flight Center. He also appreciates the generosity of the Epsilon Foundation.