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This paper reports direct evidence that some hot subdwarf stars have made very heavy elements inside themselves. The authors analysed the first far-ultraviolet spectra of two “heavy‑metal” hot subdwarfs, LSIV‑14 116 and EC22536‑5304, and found abundance patterns that point to neutron‑capture nucleosynthesis rather than only surface diffusion of material.
The team used Hubble Space Telescope (HST) spectra taken with the STIS instrument covering 1143–1730 Å at a resolving power of about R = 45,800 and a typical signal‑to‑noise ratio near 20 (programme ID 17072). They also used even higher resolution HST data of a related star and high‑resolution optical spectra from the UVES instrument (R ≈ 40,000) to help identify lines that appear only in the optical. Many of the spectral lines they needed are from ions in unusual charge states (III–VI), and these lines are missing or incomplete in standard lists.
To interpret the spectra the authors compiled and expanded atomic line data from many sources. Where data were absent they computed new oscillator strengths (a measure of line strength) for As III, Se III, Hf IV and Tl IV. They also produced new photoionisation cross sections for Pb III–VI, which allowed them to make the first non‑LTE (non‑local thermodynamic equilibrium) models of multiply ionised lead. That step matters because these hot atmospheres reach temperatures up to ~40,000 K, where simple LTE assumptions break down.
The results show very large enrichments of many heavy elements. In LSIV‑14 116 the team detected 16 light and 24 heavy metals from gallium up to bismuth (Ga–Bi). Several species — bromine, niobium, molybdenum, palladium, indium, antimony, tellurium and xenon — are reported for the first time in a hot subdwarf. EC22536‑5304 is even more enriched: 13 light and 26 heavy metals, including first detections in this class of many lanthanides and very heavy elements such as La, Ce, Pr, Nd, Er, Yb, Lu, Hf, Ta, W, Os, Pt, Hg, Tl and Bi. Quantitatively, LSIV‑14 116 peaks at about +4.3 dex (a factor ~20,000) relative to solar for Sr–Sn and is +3.1 dex at Pb and +2.3 dex at Bi. EC22536‑5304 reaches about +6.2 dex for Pb and +5.4 dex for Bi. Both stars are iron poor (−0.8 and −2.5 dex).
Crucially, the measured patterns cannot be explained by atomic diffusion alone and preserve a clear nucleosynthesis fingerprint. EC22536‑5304 in particular matches predictions for the intermediate neutron‑capture process (the i‑process), a mode of element production that operates at neutron densities between those of the slow and rapid neutron‑capture processes. That match is taken as strong evidence that at least this star self‑enriched in heavy elements during its formation. The paper links these differences to different formation histories: EC22536‑5304 likely formed by stable Roche‑lobe overflow onto its F‑type companion, while LSIV‑14 116 likely formed from the merger of two low‑mass white dwarfs.
Important caveats are noted. The ultraviolet spectra contain many lines that are absent from standard databases, so the authors had to compile and compute new atomic data; some sources or transitions may still be missing. The non‑LTE modelling and new atomic calculations are complex, and further technical details and comparisons to other theoretical methods are to be presented in follow‑up work. Finally, the paper reports two well studied stars, so broader claims about the whole class of helium‑rich hot subdwarfs remain tentative. The results do suggest, however, that self‑synthesis by neutron capture could be common in these rare stars and deserve further study.