Lead-Rich Stars Offer Clues to a Hidden Heavy-Element Factory

A chemical inheritance worth investigating

Two chemically unusual stars may preserve evidence of a heavy-element manufacturing process operating in environments where it remains poorly established. In a new preprint, Catherine Manea and colleagues identify UCAC4 278-068826 and UCAC4 179-210559 as promising candidates for enrichment by the intermediate neutron-capture process, or i-process.

Their evidence comes from stellar chemistry, not from watching nuclear reactions happen. The authors find that an existing i-process model reproduces the two stars’ measured heavy-element patterns better than any of the slow neutron-capture models they tested. Lead provides the most important distinction.

The work is available as arXiv preprint 2611.05470v1, submitted on November 3, 2026. Its landing page says it was submitted to MNRAS; the supplied version does not establish peer-reviewed publication.

Between slow and rapid neutron capture

Many elements heavier than iron form when atomic nuclei capture neutrons, with radioactive decay helping transform one element into another. The supply of neutrons shapes the resulting chemical mixture.

Astronomers traditionally distinguish the slow, or s-process, from the rapid, or r-process. The i-process occupies an intermediate neutron-density regime and can produce abundance patterns that resemble contributions from both.

One proposed setting involves hydrogen-rich material mixing into a helium-burning region inside an evolved star. This can create carbon-13, which releases neutrons through a subsequent nuclear reaction. Whether such episodes occur depends partly on how stellar models describe mixing across internal boundaries.

Much observational work has focused on stars with very low abundances of elements heavier than helium—what astronomers call low metallicity. Manea and colleagues instead investigate relatively metal-rich stars. Here, that description is comparative: the sample still has less iron relative to hydrogen than the Sun.

Reading the light of eight stars

The team studied six targets selected from the GALAH survey for unusually high reported barium and either europium or samarium abundances. They obtained follow-up spectra with Gemini South’s GHOST instrument and the Tull spectrograph at McDonald Observatory. They also reanalysed archival VLT/UVES spectra of two previously proposed i-process candidates.

These barium stars are useful chemical archives. Their enrichment is generally attributed to material transferred from a companion during its asymptotic giant branch, or AGB, phase. That former donor is now a white dwarf, while the receiving star retains traces of the transferred material.

By comparing observed absorption features with synthetic spectra, the researchers derived abundances for up to 42 elements, from lithium to lead. They also investigated carbon and magnesium isotopes, although reliable isotope constraints were possible for only part of the sample.

Lead required particular care. Its diagnostic absorption feature overlaps neighbouring atomic and molecular features. The authors fitted this region separately and applied approximate corrections for departures from local thermodynamic equilibrium, an assumption used in spectral modelling.

Why lead changes the interpretation

Three stars showed exceptionally high lead-to-iron ratios—tens to hundreds of times the solar ratio. Their chemistry resembles patterns found in some much more metal-poor, carbon-enhanced stars rather than typical metal-rich barium stars.

The authors compared the observations with several AGB model sets, accounting for transferred material becoming diluted in the receiving star’s envelope and for its initial chemical composition. For the two strongest new candidates, the i-process model gave acceptable fits, while all tested s-process alternatives fitted poorly.

That result does not make extreme lead a stand-alone verdict. The third lead-rich star, UCAC4 445-009460, marginally favoured an s-process model, although the i-process interpretation could not be excluded. The two McDonald targets also favoured s-process models, with one remaining poorly fitted. Another star’s unusual chemistry may reflect atomic diffusion, which redistributes elements, rather than the tested AGB enrichment scenarios.

The two previously published candidates remained compatible with i-process enrichment, but s-process models could also explain them. Their archival spectra lacked the lead feature needed for a potentially decisive comparison.

Strong candidates, limited coverage

The central limitation is theoretical: only one available near-solar-metallicity i-process model produced enough heavy-element enrichment for the comparison. It represents a specific stellar mass, metallicity and mixing prescription, not a broad exploration of possible i-process conditions.

The statistical comparisons also assume uncorrelated Gaussian uncertainties. The fits use a selected subset of measured elements, excluding some because of unreliable predictions or problematic spectral measurements. Corrections beyond local thermodynamic equilibrium were unavailable for many elements at these extreme enrichments.

This deliberately selected eight-star sample cannot establish how common i-process enrichment is across the Milky Way. Nor does it directly reveal the original donor’s internal mixing event.

What it does provide is a focused observational target: two stars whose lead-rich patterns challenge the conventional models tested. Broader i-process calculations and additional lead measurements could help determine how much this intermediate route contributes to the Galaxy’s heavy elements.