Evidence for neutron capture in heavy-metal hot subdwarfs

Far-UV spectroscopy of EC22536–5304 and LS IV–14°116

¹ Institut für Physik und Astronomie, Universität Potsdam, Haus 28, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany
² Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, UK
³ Atomic Physics and Astrophysics, Université de Mons (UMONS), Mons, Belgium
⁴ Astronomy and Astrophysics Institute, Université Libre de Bruxelles (ULB), Brussels, Belgium
⁵ Astrophysics Research Centre, Queen’s University Belfast, Belfast BT7 1NN, Northern Ireland, UK
⁶ Instituto de Astrofísica de La Plata, CONICET, Avenida Centenario (Paseo del Bosque) S/N, B1900FWA La Plata, Argentina
⁷ Dr. Remeis-Sternwarte & ECAP, Friedrich-Alexander University Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
⁸ School of Mathematics, Statistics and Physics, Newcastle University, Newcastle upon Tyne NE1 7RU, UK

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 9 March 2026
Accepted: 20 May 2026

Abstract

Context. 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 10⁴ times solar, particularly in Zr or Pb.

Aims. We analysed the first ultraviolet spectra of the heavy-metal subdwarfs LS IV–14°116 (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–14°116, likely formed by a white dwarf merger.

Methods. Both stars show exceptionally rich heavy-element spectra dominated by ions in stages III–VI, 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 SYNSPEC. As data were unavailable, we computed the oscillator strengths for As III, Se III, Hf IV, and Tl IV. New photoionisation cross-sections for Pb III–VI enabled the first non-local thermal equilibrium (non-LTE) models of multiply ionised Pb.

Results. In LS IV–14°116 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–14°116 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).

Conclusions. 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–14°116 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.