The peculiar abundances of HE 1005-1439 A carbon-enhanced extremely metal-poor star contaminated with products of both s- and i-process nucleosynthesis

Context. Understanding the surface chemical composition of carbon-enhanced metal-poor (CEMP) stars with enhanced abundances of heavy elements remains problematic. Aims. One of the primary objectives is to investigate the origin of the peculiar abundance pattern observed in the carbon-enhanced extremely metal-poor object HE 1005-1439, which is enriched with both s-process and i-process nucleosynthesis products and thus forms a new class of object with a distinct abundance pattern. Methods. We performed a detailed, high-resolution spectroscopic analysis of this object based on SUBARU/HDS spectra with a resolution R of -50 000. We utilised the line analysis method with measured equivalent widths of neutral and ionised lines due to various elements. Moreover, we calculated the spectrum synthesis of carbon molecular bands and lines due to elements with hyperfine structures to determine the elemental abundances. Abundances of ten light elements from C through Ni and 12 heavy elements Sr, Y, Ba, La, Ce, Pr, Nd, Eu, Dy, Er, Hf, and Pb were determined. We also performed a parametric-model-based analysis of the abundances of the heavy elements to understand the origin of the observed abundance pattern. Results. For the first time, we came across an object with a surface chemical composition that exhibits contributions from both slow (s) and intermediate (i) neutron-capture nucleosynthesis. The observed abundance pattern is quite unique and has never been observed before in any CEMP stars. The star is found to be a CEMP-s star based on the CEMP stars' classification criteria. However, the observed abundance pattern could not be explained based on theoretical s-process model predictions. On the contrary, our parametricmodel based analysis clearly indicates its surface chemical composition being influenced by similar contributions from both the s- and i-process. We critically examined the observed abundances and carefully investigated the formation scenarios involving the s-process and the i-process that are available in literature, and we found that none of them could explain the observed abundances. We note that the variation we see in our radial velocity estimates obtained from several epochs may indicate the presence of a binary companion. Considering a binary system, we therefore propose a formation scenario for this object involving effective proton ingestion episodes triggering i-process nucleosynthesis followed by s-process asymptotic giant branch (AGB) nucleosynthesis with a few third-dredge-up episodes in the now extinct companion AGB star. Results obtained from the parametric-model-based analysis are discussed in light of existing stellar evolutionary models.

Automated summary

Understanding the chemical composition of CEMP stars with enhanced heavy element abundances remains problematic. In this study, we analyzed the carbon-enhanced extremely metal-poor object HE 1005-1439 and found a unique surface chemical composition that is influenced by both s-process and i-process nucleosynthesis. The observed abundance pattern cannot be explained by theoretical s-process models, but our parametric-model-based analysis suggests contributions from both s- and i-process. We propose a formation scenario involving proton ingestion episodes followed by s-process AGB nucleosynthesis in a now extinct companion AGB star.