  Studies of nucleosynthesis in neutrino-driven winds from nascent neutron
stars show that the elements from Sr through Ag with mass numbers A~88-110 are
produced by charged-particle reactions (CPR) during the alpha-process in the
winds. Accordingly, we have attributed all these elements in stars of low
metallicities ([Fe/H]<-1.5) to low-mass and normal supernovae (SNe) from
progenitors of ~8-11M_sun and ~12-25M_sun, respectively, which leave behind
neutron stars. 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. The high-resolution data now
available on Sr abundances in Galactic halo stars show that there is a great
shortfall of Sr relative to Fe in many stars with [Fe/H]<-3. This is in direct
conflict with the above prediction. The same conflict also exists for two other
CPR elements Y and Zr. The very low abundances of Sr, Y, and Zr observed in
stars with [Fe/H]<-3 thus require a stellar source that cannot be low-mass or
normal SNe. We show that this observation requires a stellar source leaving
behind black holes and that hypernovae (HNe) from progenitors of ~25-50M_sun
are the most plausible candidates. (Abridged)
