Extremely metal-poor stars. The carbon-rich, neutron capture element-poor object CS 22957-027

Analysis of high-resolution spectra of the carbon-rich, metal-poor giant CS 22957-027 yields [Fe/H] = -3.38, [C/Fe] = 2.2, C-12/C-13 = 10, [N/Fe] = 2.0, [Sr/Fe] = -0.6, and [Ba/Fe] = -1.0. This combination of large C and N overabundances and the absence of heavy neutron capture element enrichment differs from that normally found in C-rich metal-poor stars, which are all usually enhanced in these elements. Any explanation in terms of the canonical binary mass transfer origin of CH stars or of supernovae ejecta requires a decoupling of the processes that produce the C and neutron capture element overabundances. An alternative possibility is that the heavy element abundances are typical of the halo interstellar medium from which CS 22957-027 formed and the C and N overabundances result from processes internal to the star itself. One candidate process is helium core flash induced mixing in low-mass, zero heavy element stars as predicted by Hollowell et al. CS 22957-027 exhibits strong features of (CH)-C-13 at similar to 4019.0 Angstrom, which coincide with Th II 4019.12 Angstrom. This should be borne in mind when using the Th/Eu ratio in metal-poor, C-rich stars such as CS 22892-052 as a Galactic chronometer.