Evidence for neutron capture in heavy-metal hot subdwarfs: Far-UV spectroscopy of EC22536-5304 and LSIV-14 116

Abstract

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, especially in Zr or Pb. We analyse the first ultraviolet spectra of the "heavy metal" subdwarfs LSIV-14 116 (Zr-rich) and EC22536-5304 (Pb-rich) to determine their abundance patterns and test nucleosynthesis models. Both stars show exceptionally rich heavy-element spectra dominated by ions in stages III-VI, many absent from standard line lists. We compiled literature energy levels, wavelengths, and oscillator strengths and implemented them in the SYNSPEC code. In addition, we computed new oscillator strengths for As III, Se III, Hf IV, and Tl IV. New photoionisation cross-sections for Pb III-VI enabled the first non-LTE models of multiply ionised Pb. In LSIV-14 116 we detect 16 light and 24 heavy metals (Ga-Bi); Br, Nb, Mo, Pd, In, Sb, Te, and Xe are measured in an sdO/B star for the first time. In EC22536-5304 13 light and 26 heavy metals are detected, including first detections of La, Ce, Pr, Nd, Er, Yb, Lu, Hf, Ta, W, Os, Pt, Hg, Tl, and Bi. LSIV-14 116 peaks at ~4.3 dex for Sr-Sn relative to solar, declining to 3.1 dex at Pb and 2.3 dex at Bi, whereas EC22536-5304 reaches 6.2 dex for Pb and 5.4 dex for Bi. Both stars are Fe-poor. The abundance patterns cannot be explained by atomic diffusion alone and retain a clear nucleosynthetic signature. EC22536-5304 closely matches predictions of i-process nucleosynthesis, providing strong evidence for i-process self-enrichment in hot subdwarfs. EC22536-5304 likely formed via Roche-lobe overflow, whereas LSIV-14 116 likely originated from the merger of two low-mass white dwarfs, which may explain differences in its enrichment pattern. These results suggest that heavy metals in other He-sdO/Bs may also be self-synthesised.