The Abundance of Lead in Four Metal-Poor Stars
  Cowan et al. (2021) review how roughly half the elements heavier than iron
found in the Sun are produced by rapid neutron capture and half by slow neutron
capture, the r- and s-processes. In the Sun, their relative contribution to
individual elemental abundances is well understood, except for the lightest and
heaviest elements beyond iron. Their contributions are especially uncertain for
the heaviest non-radioactive element, lead (Pb, Z=82). This is constrained by
deriving lead abundances in metal-poor stars. For in the most metal-poor halo
stars, strontium and heavier elements are found in the solar r-process
proportion; s-process elements appear only at metallicities above one-thirtieth
solar.
  In unevolved metal-poor stars of roughly solar heavy-element content, only
two UV Pb lines are detectable. Four such stars have high-resolution spectra of
the strongest line, Pb II at 2203.53A. Roederer et al. (2020) analyzed this
line in one star, deriving a lead-to-iron abundance ratio ten times solar. This
and its blue-shifted profile suggested strong s-process production.
  This work analyzes the UV spectra of all four stars. Calculations including a
predicted Fe I line blueward of the Pb II line, and assuming the lead abundance
scales with r-process abundances, match all four profiles extremely well. A
scaled s-process contribution might improve the match to the much lower lead
abundance found in the unevolved star analyzed previously, but its s-process
excess is modest. An Fe II line blends the other lead line, Pb I at 2833.05A,
which constrains the lead abundance only in the coolest star.
