The Abundance of Lead in Four Metal-Poor Stars

Ruth C. Peterson

Published 2021-05-20Version 1

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.