Abundances of neutron-capture elements in CH and carbon-enhanced metal-poor (CEMP) stars

Abstract

All the elements heavier than Fe are produced either by the slow (-s) or rapid (-r) neutron-capture process. The neutron density prevailing in the stellar sites is one of the major factors that determines the type of neutron-capture processes. We present the results based on the estimates of corrected value of absolute carbon abundance, [C/N] ratio, carbon isotopic ratio and [hs/ls] ratio obtained from the high-resolution spectral analysis of six stars that include both CH stars and CEMP stars. All the stars show enhancement of neutron-capture elements. Location of these objects in the A(C) vs. [Fe/H] diagram shows that they are Group I objects, with external origin of carbon and neutron-capture elements. Low values of carbon isotopic ratios estimated for these objects may also be attributed to some external sources. As the carbon isotopic ratio is a good indicator of mixing, we have used the estimates of 12C/13C ratios to examine the occurrence of mixing in the stars. While the object HD 30443 might have experienced an extra mixing process that usually occurs after red giant branch (RGB) bump for stars with log(L/L⊙) > 2.0, the remaining objects do not show any evidence of having undergone any such mixing process. The higher values of [C/N] ratios obtained for these objects also indicate that none of these objects have experienced any strong internal mixing processes. Based on the estimated abundances of carbon and the neutron-capture elements, and the abundance ratios, we have classified the objects into different groups. While the objects HE 0110−0406, HD 30443 and CD−38 2151 are found to be CEMP-s stars, HE 0308−1612 and HD 176021 show characteristic properties of CH stars with moderate enhancement of carbon. The object CD−28 1082 with enhancement of both r- and s-process elements is found to belong to the CEMP-r/s group.