Abundances of Sr, Y, and ZR in metal-poor stars and implications for chemical evolution in 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 (CPRs) in neutrino-driven winds, which are associated with neutron star formation in low-mass and normal supernovae (SNe) from progenitors of ∼8Y11 M _⊙ and ∼12-25 M_⊙, 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 ∼25-50 M _⊙ 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.