The chemical composition of carbon-rich, very metal poor stars: A new class of mildly carbon rich objects without excess of neutron-capture elements

We report on an analysis of the chemical composition of five carbon-rich, very metal poor stars based on high-resolution spectra. One star, CS 22948-027, exhibits very large overabundances of carbon, nitrogen, and the neutron-capture elements, as found in the previous study of Hill et al. This result can be interpreted as a consequence of mass transfer from a binary companion that previously evolved through the asymptotic giant branch stage. By way of contrast, the other four stars we investigate exhibit no overabundances of barium ([Ba/Fe] < 0), while three of them have mildly enhanced carbon and/or nitrogen ([C + N] ∼ + 1). We have been unable to determine accurate carbon and nitrogen abundances for the remaining star (CS 30312-100). These stars are rather similar to the carbon-rich, neutron-capture-element-poor star CS 22957-027 discussed previously by Norris et al., although the carbon over-abundance in this object is significantly larger ([C/Fe] = + 2.2). Our results imply that these carbon-rich objects with "normal" neutron-capture element abundances are not rare among very metal-deficient stars. One possible process to explain this phenomenon is as a result of helium-shell flashes near the base of the asymptotic giant branch in very low metallicity, low-mass (M ≲ 1M_⊙) stars, as recently proposed by Fujimoto et al. The moderate carbon enhancements reported here ([C/Fe] ∼ + 1) are similar to those reported in the famous r-process-enhanced star CS 22892-052. We discuss the possibility that the same process might be responsible for this similarity, as well as the implication that a completely independent phenomenon was responsible for the large r-process enhancement in CS 22892-052.