i-process nucleosynthesis: observational evidences from CEMP stars

Abstract: The surface chemical compositions of a large fraction of Carbon-Enhanced Metal-Poor (CEMP) stars, the so-called CEMP-r/s stars, are known to exhibit enhancement of both s- and r-process elements. For these stars, the heavy element abundances cannot be explained either by s-process or r-process nucleosynthesis alone, as the production sites of s- and r-process elements are very different, and these two processes produce distinct abundance patterns. Thus, the observational evidence of the double enhancement seen in CEMP-r/s stars remains a puzzle as far as the origin of the elements is concerned. In this work, we have critically analysed the observed abundances of heavy elements in a sample of eight CEMP-r/s stars from literature, to trace the origin of the observed double enhancement. Towards this, we have conducted a parametric-model based analysis to delineate the contributions of s- and r-process nucleosynthesis to the observed elemental abundances. We have further examined if the i-process (intermediate process) nucleosynthesis, that occurs at a high neutron-density (n$\sim$10$^{15}$ cm$^{-3}$ ) produced during proton-ingestion from a H-rich envelope to the intershell region of an AGB star, that is capable of producing both r- and s-process elements in a single stellar site, could explain the observed abundance patterns of the sample stars. Our analysis shows that the observed abundance patterns of the selected sample of CEMP-r/s stars could be fairly well reproduced using the i-process model yields.