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Studies of nucleosynthesis in neutrino-driven winds from nascent neutronstars show that the elements from Sr through Ag with mass numbers A~88-110 areproduced by charged-particle reactions (CPR) during the alpha-process in thewinds. Accordingly, we have attributed all these elements in stars of lowmetallicities ([Fe/H]<-1.5) to low-mass and normal supernovae (SNe) fromprogenitors of ~8-11M_sun and ~12-25M_sun, respectively, which leave behindneutron stars. Using this rule and attributing all Fe production to normal SNe,we previously developed a phenomenological two-component model, which predictsthat [Sr/Fe]>-0.32 for all metal-poor stars. The high-resolution data nowavailable on Sr abundances in Galactic halo stars show that there is a greatshortfall of Sr relative to Fe in many stars with [Fe/H]<-3. This is in directconflict with the above prediction. The same conflict also exists for two otherCPR elements Y and Zr. The very low abundances of Sr, Y, and Zr observed instars with [Fe/H]<-3 thus require a stellar source that cannot be low-mass ornormal SNe. We show that this observation requires a stellar source leavingbehind black holes and that hypernovae (HNe) from progenitors of ~25-50M_sunare the most plausible candidates. (Abridged)

Abundances of Sr, Y, and Zr in Metal-Poor Stars and Implications for Chemical Evolution in the Early Galaxy