Context. The origin of the enhanced abundances of both s- and r-process elements observed in a subclass of carbon-enhanced metal-poor (CEMP) stars, denoted CEMP-r/s stars, still remains poorly understood. The i-process nucleosynthesis has been suggested as one of the most promising mechanisms for the origin of these stars. Aims. Our aim is to better understand the chemical signatures and formation mechanism(s) of five previously claimed potential CH star candidates HE 0017+0055, HE 2144−1832, HE 2339−0837, HD 145777, and CD−27 14351 through a detailed systematic follow-up spectroscopic study based on high-resolution spectra. Methods. The stellar atmospheric parameters, the effective temperature Teff, the microturbulent velocity ζ, the surface gravity log g, and the metallicity [Fe/H] are derived from local thermodynamic equilibrium analyses using model atmospheres. Elemental abundances of C, N, α-elements, iron-peak elements, and several neutron-capture elements are estimated using the equivalent width measurement technique as well as spectrum synthesis calculations in some cases. In the context of the double enhancement observed in four of the programme stars, we have critically examined whether the literature i-process model yields ([X/Fe]) of heavy elements can explain the observed abundance distribution. Results. The estimated metallicity [Fe/H] of the programme stars ranges from −1.63 to −2.74. All five stars show enhanced abundance for Ba, and four of them exhibit enhanced abundance for Eu. Based on our analysis, HE 0017+0055, HE 2144−1832, and HE 2339−0837 are found to be CEMP-r/s stars, whereas HD 145777 and CD−27 14351 show characteristic properties of CEMP-s stars. From a detailed analysis of different classifiers of CEMP stars, we have identified the one which best describes the CEMP-s and CEMP-r/s stars. We found that for both CEMP-s and CEMP-r/s stars, [Ba/Eu] and [La/Eu] exhibit positive values and [Ba/Fe] ≥ 1.0. However, CEMP-r/s stars satisfy [Eu/Fe] ≥ 1.0, 0.0 ≤ [Ba/Eu] ≤ 1.0, and/or 0.0 ≤ [La/Eu] ≤ 0.7. CEMP-s stars normally show [Eu/Fe] < 1.0 with [Ba/Eu] > 0.0 and/or [La/Eu] > 0.5. If [Eu/Fe] ≥ 1.0, then the condition on [Ba/Eu] and/or [La/Eu] for a star to be a CEMP-s star is [Ba/Eu] > 1.0 and/or [La/Eu] > 0.7. Using a large sample of similar stars from the literature we have examined whether the ratio of heavy-s to light-s process elements [hs/ls] alone can be used as a classifier, and if there are any limiting values for [hs/ls] that can be used to distinguish between CEMP-s and CEMP-r/s stars. Even though they peak at different values of [hs/ls], CEMP-s and CEMP-r/s stars show an overlap in the range 0.0 < [hs/ls] < 1.5, and hence this ratio cannot be used to distinguish between CEMP-s and CEMP-r/s stars. We have noticed a similar overlap in the case of [Sr/Ba] as well, in the range −1.6 < [Sr/Ba] < −0.5, and hence this ratio also cannot be used to separate the two subclasses.

Barium stars are one of the important probes to understand the origin and evolution of slow neutron-capture process elements in the Galaxy. These are extrinsic stars, where the observed s-process element abundances are believed to have an origin in the now invisible companions that produced these elements at their asymptotic giant branch (AGB) phase of evolution. We have attempted to understand the s-process nucleosynthesis, as well as the physical properties of the companion stars through a detailed comparison of observed elemental abundances of 10 barium stars with the predictions from AGB nucleosynthesis models, FRUITY. For these stars, we have presented estimates of abundances of several elements, C, N, O, Na, Al, α-elements, Fe-peak elements, and neutron-capture elements Rb, Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, and Eu. The abundance estimates are based on high resolution spectral analysis. Observations of Rb in four of these stars have allowed us to put a limit to the mass of the companion AGB stars. Our analysis clearly shows that the former companions responsible for the surface abundance peculiarities of these stars are low-mass AGB stars. Kinematic analysis has shown the stars to be members of Galactic disc population.

In this survey paper, we present open problems and conjectures on visibility graphs of points, segments and polygons along with necessary backgrounds for understanding them.

Elemental abundances of CH stars can provide observational constraints for theoretical studies on the nucleosynthesis and evolution of low-and intermediate-mass stars. However, available abundance data in literature are quite scanty. In our pursuit to generate a homogeneous database of elemental abundances of CH stars we have performed a detailed chemical abundance study for a sample of 12 potential CH star candidates based on high resolution spectroscopy. We present first time abundance analysis for the objects HE0308−1612, CD−281082, HD30443, and HD87853. For the other objects, although limited information is available, detailed chemical composition studies are missing. Our analysis shows CD−281082 to be a very metal-poor object with [Fe/H]=−2.45 and enriched in carbon with [C/Fe] = 2.19. With a ratio of [Ba/Eu]∼0.02 the star satisfies the classification criteria of a CEMP-r/s star. The objects CD−382151 with [Fe/H]=−2.03 and HD30443 with [Fe/H]∼−1.68 are found to show the characteristic properties of CH stars. HE0308−1612 and HD87853 are found to be moderately metal-poor with [Fe/H]∼−0.73; while HE0308−1612 is moderately enhanced with carbon ([C/Fe]∼0.78) and shows the spectral properties of CH stars, the abundance of carbon could not be estimated for HD87853. Among the two moderately metal-poor objects, HD87080 ([Fe/H]=−0.48) shows near solar carbon abundance, and HD176021 ([Fe/H]=−0.63 ) is mildly enhanced in carbon with [C/Fe]=0.52. HD176021 along with HD202020 a known binary, exhibit the characteristic properties of CH stars as far as the heavy element abundances are concerned. Five objects in our sample show spectral properties that are normally seen in barium stars.