| Abstract: Understanding the origin of heavy-element enrichment in carbon-enhanced metal-poor (CEMP) stars remains one of the most persistent and unresolved problems in stellar nucleosynthesis and early Galactic chemical evolution. In this study, we report a groundbreaking discovery identifying HE 1005-1439 as the first known star to provide observational evidence for the coexistence of s-process and i-process nucleosynthesis products within a single star, marking a previously unrecognized class of CEMP stars. A high-resolution spectroscopic analysis was conducted on high-quality SUBARU/HDS spectra of the object at a resolving power of approximately 50,000. We precisely estimated the abundances of ten light elements from C to Ni and twelve neutron-capture elements, including Sr, Y, Ba, La, Ce, Pr, Nd, Eu, Dy, Er, Hf, and Pb. The derived abundance pattern of the object could neither be explained based on the classical s-process predictions nor with the help of i-process models alone. We have performed a detailed parametric-model-based analysis and found that the star is contaminated by the products of both s- and i-processes with similar contributions from both processes. We also note that the radial velocity of the object, obtained from several epochs, has revealed the presence of a binary companion. We, therefore, propose a novel formation scenario for the object in which proton-ingestion episodes in a low-metallicity asymptotic giant branch (AGB) companion trigger i-process nucleosynthesis, followed by subsequent s-process enrichment during later AGB evolution with a few third dredge-up episodes and mass transfer to the observed star. This star is expected to fill an important gap in our understanding of the link between the s- and i-processes. |
