Nucleosynthesis in low-mass stars: Shedding light on the cosmic origin of heavy elements

Abstract

Low-mass, low-metallicity stars while evolving through the asymptotic giant branch (AGB) phase undergo significant nucleosynthesis producing the majority of slow (s-) and intermedite (i-) neutron capture elements. To understand the contribution of low and intermediate-mass stars to the Galactic chemical enrichment it is therefore important to account for the products of AGB nucleosynthesis. High resolution optical spectroscopic studies of stars in the AGB phase are, however, quite challenging due to obscuration by surrounding dust envelopes. The carbon enhanced metal-poor (CEMP) stars ([C/Fe] ≥ 0.7), with enhanced abundances of neutron-capture elements, namely the CH, CEMP-s, CEMP-r/s, CEMP-i stars are known to be mostly binaries. The overabundance of carbon and the heavy elements observed in these stars are believed to have originated in a former AGB companion and subsequently transferred to the stars. The surface chemical composition of these stars can therefore be used to get insight into the origin and evolution of heavy elements and some poorly-understood aspects of AGB nucleosynthesis, such as, mixing of protons in C-rich layers that is needed to produce the required neutron source for neutron-capture nucleosynthesis. The CEMP-no group with no evidence of overabundance of neutron-capture elements dominate the population at the lowest metallicities and are mostly found to be single stars. The abundances of heavy neutron-capture elements as produced in their unseen precursors reveal important characteristics of first stellear populations. In the past few years, we have studied a large sample of Ba, CH, CEMP-s, CEMP-r/s and CEMPno stars based on high resolution spectroscopy to better understand the role of low-mass stars in the chemical enrichment of the Galaxy. In this talk, some highlights of the results obtained from these studies are presented. Our efforts towards providing an improved classification scheme for accurate classification of CEMP-s and CEMP-r/s stars and in characterizing the companion asymptotic giant branch (AGB) stars of CH, CEMP-no, CEMP-s and CEMP-r/s binary systems are briefly discussed.