| Abstract : | The elements heavier than Fe are believed to be produced mainly by slow (s-) and rapid (r-) neutron-capture processes. While the production sites of s-process elements are believed to be the inter-pulse phases of low-mass AGB stars, the r-process that requires very high temperatures and neutron fluxes is expected to occur during supernova explosions and neutron-star mergers. It is seen that these two processes are not enough to explain the observed heavy element abundances in many CEMP-r/s stars. In order to explain the abundance patterns of these stars, an ‘intermediate’ (i-) process has recently been suggested. For the first time, we came across an object, HE1005-1439, with a surface chemical composition that exhibits contributions from both s- and i-processes. We performed a detailed, high-resolution spectroscopic analysis of HE1005-1439 based on SUBARU/HDS spectra. Abundances of ten light elements from C through Ni and twelve heavy elements from Sr through Pb were determined. The observed abundance pattern is unique and has never been observed before. The observed abundance pattern could not be explained based on theoretical s-process or i-process model predictions alone. Parametric-model-based analysis clearly indicates its surface chemical composition being influenced by similar contributions from both the s- and i-processes. We found that none of the existing formation scenarios in the literature involving s-process and i-process 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. We, therefore, proposed a formation scenario for this object involving effective proton ingestion episodes (PIEs) triggering i-process nucleosynthesis followed by s-process asymptotic giant branch (AGB) nucleosynthesis in the now extinct companion AGB star. Results obtained from the parametric-model-based analysis will be discussed in light of existing stellar evolutionary models. |
