Searching for Signatures of i-process Nucleosynthesis in Metal-Rich, Lead-rich Barium Stars

Abstract

The intermediate neutron-capture process ($i$-process) has been invoked to explain the peculiar compositions of carbon-enhanced metal-poor (CEMP) stars enriched in both slow- ($s$) and rapid- ($r$) neutron-capture elements. Recent literature has questioned its operation at higher metallicities, but observational evidence remains limited. We analyse Gemini/GHOST and McDonald/Tull spectra of six $-0.7<[Fe/H]<-0.1$ stars identified in GALAH by their extreme Ba and Eu (or Sm) compositions ($\gtrsim$10$\times$ solar). We also reanalyse archival spectra of two literature $i$-process candidates. We determine abundances in up to 42 elements (Li through Pb), ${}^{12}$C/${}^{13}$C, and Mg isotopic ratios, and compare to literature barium and CEMP stars and multiple asymptotic giant branch (AGB) model grids. Three stars exhibit extreme [Pb/Fe] (+1.79$\pm$0.19 to +2.50$\pm$0.32 dex), and two extreme [Eu/Fe] (+1.10$\pm$0.11 to +1.30$\pm$0.11 dex), distinguishing them from typical metal-rich barium stars. Two of these three exhibit abundance patterns that are inconsistent with $s$-process models and best explained by existing $i$-process predictions. The remaining sample is either consistent with $s$-process enrichment or suggestive of atomic diffusion. Both literature candidates are well-fit by the $i$-process model, but conventional $s$-process enrichment cannot be excluded. Our findings reinforce that metal-rich AGB stars contribute to the production of elements classically attributed to the $r$-process. These results identify promising candidates for $i$-process enrichment near solar metallicity and motivate additional $i$-process models to understand the contribution of this nucleosynthetic process to the chemical evolution of the Milky Way disk.

arXiv comment18 pages, 7 figures, 7 tables. Submitted to MNRAS. Comments welcome