Chemical Evolution of the Juvenile Universe
  Only massive stars contribute to the chemical evolution of the juvenile
universe corresponding to [Fe/H]<-1.5. If Type II supernovae (SNe II) are the
only relevant sources, then the abundances in the interstellar medium of the
juvenile epoch are simply the sum of different SN II contributions. Both
low-mass (~8-11M_sun) and normal (~12-25M_sun) SNe II produce neutron stars,
which have intense neutrino-driven winds in their nascent stages. These winds
produce elements such as Sr, Y, and Zr through charged-particle reactions
(CPR). Such elements are often called the light r-process elements, but are
considered here as products of CPR and not the r-process. The observed absence
of production of the low-A elements (Na through Zn including Fe) when the true
r-process elements (Ba and above) are produced requires that only low-mass SNe
II be the site if the r-process occurs in SNe II. Normal SNe II produce the CPR
elements in addition to the low-A elements. This results in a two-component
model that is quantitatively successful in explaining the abundances of all
elements relative to hydrogen for -3<[Fe/H]<-1.5. This model explicitly
predicts that [Sr/Fe]>-0.32. Recent observations show that there are stars with
[Sr/Fe]<-2 and [Fe/H]<-3. This proves that the two-component model is not
correct and that a third component is necessary to explain the observations.
This leads to a simple three-component model including low-mass and normal SNe
II and hypernovae (HNe), which gives a good description of essentially all the
data for stars with [Fe/H]<-1.5. We conclude that HNe are more important than
normal SNe II in the chemical evolution of the low-A elements, in sharp
distinction to earlier models. (Abridged)
