The Chemical Composition of Carbon-Rich, Very Metal-Poor Stars: A New Class of Mildly Carbon-Rich Objects Without Excess of Neutron-Capture Elements

Abstract

We report on an analysis of the chemical composition of five carbon-rich, very metal-poor stars based on high-resolution spectra. One star, CS22948-027, exhibits very large overabundances of carbon, nitrogen, and the neutron-capture elements, as found in the previous study of Hill et al.. This result may 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 (CS30312-100). These stars are rather similar to the carbon-rich, neutron-capture-element-poor star CS22957-027 discussed previously by Norris et al., though the carbon overabundance 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 AGB in very low-metallicity, low-mass (M~< 1M_sun) stars, as recently proposed by Fujimoto et al.. The moderate carbon enhancements reported herein ([C/Fe]+1) are similar to those reported in the famous r-process-enhanced star CS22892-052. 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 CS22892-052.