Lead ( Pb ) is predominantly produced by the slow neutron-capture process ( s process ) in asymptotic giant branch ( AGB ) stars .
In contrast to significantly enhanced Pb abundances predicted by low-mass , low-metallicity AGB-models , observations of Magellanic post-AGB stars show incompatibly low Pb abundances .
Observations of carbon-enhanced metal-poor ( CEMP ) stars whose s-process enrichments are accompanied by heavy elements traditionally associated with the rapid neutron-capture process ( r process ) have raised the need for a neutron-capture process operating at neutron densities intermediate to the s and r process : the so-called i process .
We study i-process nucleosynthesis with single-zone nuclear-network calculations .
Our i-process models can explain the heavy-element abundance patterns measured in Magellanic post-AGB stars including their puzzlingly low Pb abundances .
Furthermore , the heavy-element enhancements in the post-AGB and CEMP-i stars , particularly their Pb abundance , allow us to characterise the neutron densities and exposures of the i process that produced the observed abundance patterns .
We find that the lower-metallicity CEMP-i stars ( \left [ \mathrm { Fe } / \mathrm { H } \right ] \approx - 2.5 ) have heavy-element abundances best matched by models with higher neutron densities and exposures ( \tau > 2.0 \mathrm { mbarn } ^ { -1 } ) compared to the higher-metallicity post-AGB stars ( \left [ \mathrm { Fe } / \mathrm { H } \right ] \approx - 1.3 , \tau < 1.3 \mathrm { mbarn } ^ { -1 } ) .
This offers new constraints and insights regarding the properties of i-process sites and demonstrates that the responsible process operates on time scales of the order of a few years or less .