Nucleosynthesis signatures of neutrino-driven winds from proto-neutron
  stars: a perspective from chemical evolution models
  We test the hypothesis that the observed first-peak (Sr, Y, Zr) and
second-peak (Ba) s-process elemental abundances in low-metallicity Milky Way
stars, and the abundances of the elements Mo and Ru, can be explained by a
pervasive r-process contribution originating in neutrino-driven winds from
highly-magnetic and rapidly rotating proto-neutron stars (proto-NSs). We
construct chemical evolution models that incorporate recent calculations of
proto-NS yields in addition to contributions from AGB stars, Type Ia
supernovae, and two alternative sets of yields for massive star winds and
core-collapse supernovae. For non-rotating massive star yields from either set,
models without proto-NS winds underpredict the observed s-process peak
abundances by $0.3$-$1\,\text{dex}$ at low metallicity, and they severely
underpredict Mo and Ru at all metallicities. Models incorporating wind yields
from proto-NSs with spin periods $P \sim 2$-$5\,\text{ms}$ fit the observed
trends for all these elements well. Alternatively, models omitting proto-NS
winds but adopting yields of rapidly rotating massive stars, with $v_{\rm rot}$
between $150$ and $300\,\text{km}\,\text{s}^{-1}$, can explain the observed
abundance levels reasonably well for $\text{[Fe/H]}<-2$. These models
overpredict [Sr/Fe] and [Mo/Fe] at higher metallicities, but with a tuned
dependence of $v_{\rm rot}$ on stellar metallicity they might achieve an
acceptable fit at all [Fe/H]. If many proto-NSs are born with strong magnetic
fields and short spin periods, then their neutrino-driven winds provide a
natural source for Sr, Y, Zr, Mo, Ru, and Ba in low-metallicity stellar
populations. Conversely, spherical winds from unmagnetized proto-NSs
overproduce the observed Sr, Y, and Zr abundances by a large factor.
