Carbon-Enhanced Metal-Poor Stars. III. Main-Sequence Turnoff Stars from the SDSS SEGUE Sample

Abstract

The chemical compositions of seven CEMP turnoff stars are determined from high-resolution spectroscopy. Five are selected from the SDSS SEGUE sample of metal-poor stars. Another star chosen from the SDSS SEGUE sample has only a weak upper limit on its carbon abundance obtained from the high-resolution spectrum. The effective temperatures are all higher than 6000 K, while the metallicities, parameterized by [Fe/H], are all below -2; the star with the lowest iron abundance has [Fe/H] = -3.1. Six of our objects exhibit high abundance ratios of barium ([Ba/Fe] > +1), suggesting large contributions of the products of former AGB companions via mass transfer across binary systems. One star (SDSS 1707+58) exhibits rapid variation in radial velocity, a strong signature that it belongs to a close binary. Combining our results with previous studies provides a total of 20 CEMP main-sequence turnoff stars for which the abundances of carbon and at least some neutron-capture elements are determined. The [C/H] ratios for this sample of CEMP turnoff stars are generally higher than those of CEMP giants and their dispersion is also smaller. These results indicate that the carbon-enhanced material from the companion AGB star is preserved at the surface of turnoff stars with no significant dilution, counter to expectations if processes such as thermohaline mixing have operated in unevolved CEMP stars. In contrast to the behavior of [C/H], a large dispersion in the observed [Ba/H] is found for the CEMP turnoff stars, suggesting that the efficiency of the s-process in very metal-poor AGB stars may differ greatly from star to star. Four of the six stars from the sample exhibit kinematics associated with membership in the outer-halo population, a remarkably high fraction.