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, CS22948-027,exhibits very large overabundances of carbon, nitrogen, and the neutron-captureelements, as found in the previous study of Hill et al.. This result may beinterpreted as a consequence of mass transfer from a binary companion thatpreviously evolved through the asymptotic giant branch stage. By way ofcontrast, the other four stars we investigate exhibit no overabundances ofbarium ([Ba/Fe]<0), while three of them have mildly enhanced carbon and/ornitrogen ([C+N]+1). We have been unable to determine accurate carbon andnitrogen abundances for the remaining star (CS30312-100). These stars arerather similar to the carbon-rich, neutron-capture-element-poor starCS22957-027 discussed previously by Norris et al., though the carbonoverabundance in this object is significantly larger ([C/Fe]=+2.2). Our resultsimply that these carbon-rich objects with ``normal'' neutron-capture elementabundances are not rare among very metal-deficient stars. One possible processto explain this phenomenon is as a result of helium shell flashes near the baseof the AGB in very low-metallicity, low-mass (M~< 1M_sun) stars, as recentlyproposed by Fujimoto et al.. The moderate carbon enhancements reported herein ([C/Fe]+1) are similar tothose reported in the famous r-process-enhanced star CS22892-052. We discussthe possibility that the same process might be responsible for this similarity,as well as the implication that a completely independent phenomenon wasresponsible for the large r-process enhancement in CS22892-052.Comment: 53 pages, 8 figures, to appear in Ap