Spectroscopic Study of Four Metal-poor Carbon Stars from the Hamburg/ESO Survey: On Confirming the Low-mass Nature of Their Companions

Elemental abundances of extrinsic carbon stars provide insight into the poorly understood origin and evolution of elements in the early Galaxy. In this work, we present the results of a detailed spectroscopic analysis of four potential carbon star candidates from the Hamburg/ESO Survey (HES): HE 0457-1805, HE 0920-0506, HE 1241-0337, and HE 1327-2116. This analysis is based on the high-resolution spectra obtained with Mercator/HERMES (R ~ 86,000) and SUBARU/HDS (R ~ 50,000). Although the abundances of a few elements, such as Fe, C, and O, are available from medium-resolution spectra, we present the first ever detailed high-resolution spectroscopic analysis for these objects. The objects HE 0457-1805 and HE 1241-0337 are found to be CEMP-s stars, HE 0920-0506 a CH star, and HE 1327-2116 a CEMP-r/s star. The object HE 0457-1805 is a confirmed binary, whereas the binary status of the other objects is unknown. The locations of program stars on the diagram of absolute carbon abundance A(C) versus [Fe/H] point at their binary nature. We have examined various elemental abundance ratios of the program stars and confirmed the low-mass nature of their former AGB companions. We have shown that the i-process models could successfully reproduce the observed abundance pattern in HE 1327-2116. The analysis performed for HE 0457-1805, HE 0920-0506, and HE 1241-0337 based on the FRUITY parametric models confirmed that the surface chemical compositions of these three objects are influenced by pollution from low-mass AGB companions. ^* Based on data collected using Mercator/HERMES, SUBARU/HDS.