Chemical Evolution of the Juvenile Universe
G. J. Wasserburg, Y.-Z. Qian

TL;DR
This paper develops models of chemical evolution in the early universe, showing that hypernovae play a crucial role alongside supernovae in producing elements observed in ancient stars.
Contribution
It introduces a three-component model including low-mass SNe II, normal SNe II, and hypernovae, improving understanding of element abundances at low metallicities.
Findings
Two-component model fails to explain low Sr/Fe stars.
Inclusion of hypernovae improves model fit to observed data.
Hypernovae are more significant than normal SNe II in early chemical enrichment.
Abstract
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…
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