High-resolution spectroscopic observations of the new CEMP-s star CD −50°776

Carbon enhanced metal poor (CEMP) stars are a particular class of low metalicity halo stars whose chemical analysis may provide important constrains to the chemistry evolution of the Galaxy and to the models of mass transfer and evolution of components in binary systems. Here, we present a detailed analysis of the CEMP star CD-50$^\circ$776, using high resolution optical spectroscopy. We found that CD-50$^\circ$776 has a metalicity [Fe/H]=-2.31 and a carbon abundance [C/Fe]=+1.21. Analyzing the s-process elements and the europium abundances, we show that this star is actually a CEMP-s star, based on the criteria set in the literature to classify these chemically peculiar objects. We also show that CD-50$^\circ$776 is a lead star, since it has a ratio [Pb/Ce]=+0.97. In addition, we show that CD-50$^\circ$776 develops radial velocity variations that may be attributed to the orbital motion in a binary system. The abundance pattern of CD-50$^\circ$776 is discussed and compared to other CEMP-s stars already reported in the literature to show that this star is a quite exceptional object among the CEMP stars, particularly due to its low nitrogen abundance. Explaining this pattern may require to improve the nucleosynthesis models, and the evolutionary models of mass transfer and binary interaction.