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Subaru/HDS study of CH stars: elemental abundances for stellar neutron-capture process studies

A comprehensive abundance analysis providing rare insight into the chemical history of lead stars is still lacking. We present results from high resolution (R ~ 50000), spectral analyses of three CH stars, HD 26, HD 198269, HD 224959, and, a carbon star with a dusty envelope, HD 100764. Previous studies on these objects are limited by both resolution and wavelength regions and the results differ significantly from each other. We have undertaken to re-analyse the chemical composition of these objects based on high resolution Subaru spectra covering the wavelength regions 4020 to 6775 A,. Considering local thermodynamic equilibrium and using model atmospheres, we have derived the stellar parameters, the effective temperatures Teff, surface gravities log g, and metallicities [Fe/H] for these objects. The derived parameters for HD 26, HD 100764, HD 198269 and HD 224959 are (5000, 1.6, -1.13), (4750, 2.0 -0.86), (4500, 1.5, -2.06) and (5050, 2.1, -2.44) respectively. The stars are found to exhibit large enhancements of heavy elements relative to iron in conformity to previous studies. Large enhancement of Pb with respect to iron is also confirmed. Updates on the elemental abundances for several s-process elements (Y, Zr, La, Ce, Nd, Sm, Pb) along with the first-time estimates of abundances for a number of other heavy elements (Sr, Ba, Pr, Eu, Er, W) are reported. Our analysis suggests that neutron-capture elements in HD 26 primarily originate in s-process while the major contributions to the abundances of neutron-capture elements in the more metal-poor objects HD 224959 and HD 198269 are from r-process, possibly formed from materials that are pre-enriched with products of r-process.