Abstract :
[en] Most hot subdwarfs (sdO/B) are low-mass core-helium-burning stars formed through binary interaction. A subgroup of intermediate He-rich sdOBs shows extreme heavy metal (Z>30) enrichments exceeding 10^4 times solar, particularly in Zr or Pb.
We analysed the first ultraviolet spectra of the heavy-metal subdwarfs LS,IV-14degr116 (Zr-rich) and EC,22536-5304 (Pb-rich) to determine their abundance patterns and test nucleosynthesis models. The targets probe contrasting evolutionary channels: the close binary EC,22536-5304 with an sdF companion and the apparently single LS,IV-14degr116, likely formed by a white dwarf merger.
Both stars show exceptionally rich heavy-element spectra dominated by ions in stages , many transitions being absent from standard line lists. We compiled literature energy levels, wavelengths, and oscillator strengths and implemented them in the spectral synthesis code . As data were unavailable, we computed the oscillator strengths for , , , and . New photoionisation cross-sections for enabled the first non-local thermal equilibrium (non-LTE) models of multiply ionised Pb. iii-vi Synspec As iii Se iii Hf iv Tl iv Pb iii-vi
In LS,IV-14degr116 we detected 16 light and 24 heavy metals (Ga-Bi); Br, Nb, Mo, Pd, In, Sb, Te, and Xe have been measured in an sdO/B star for the first time. EC,22536-5304 is even more enriched; we detected 13 light and 26 heavy metals, including first detections of La, Ce, Pr, Nd, Er, Yb, Lu, Hf, Ta, W, Os, Pt, Hg, Tl, and Bi, mainly in stages iv-v . LS,IV-14degr116 peaks at ∼4.3 dex for Sr-Sn, declining to 3.1 dex at Pb and 2.3 dex at Bi, whereas EC,22536-5304 shows 1-3 dex enhancements from Ga-Sn, rising to ∼4 dex for La-Tl and reaching 6.2 dex for Pb and 5.4 dex for Bi. Both stars are Fe-poor (-0.8 and -2.5 dex).
The photospheres of both stars are strongly enriched in heavy elements relative to their initial compositions. The abundance patterns cannot be explained by atomic diffusion alone and retain a clear signature of nucleosynthesis. The distribution in EC,22536-5304 closely matches predictions of i-process nucleosynthesis during its formation, providing strong evidence for i-process self-enrichment in hot subdwarfs. The contrasting abundance patterns imply different evolutionary histories and i-process conditions: EC,22536-5304 likely formed via Roche-lobe overflow onto its sdF-type companion, whereas the single star LS,IV-14degr116 most likely originated from the merger of two low-mass white dwarfs. These results suggest that heavy metals in other He-rich sdOB and sdO stars are also self-synthesised and that i-process nucleosynthesis may be widespread among He-rich hot subdwarfs.
