Carbon-Enhanced Metal-Poor Stars. III. Main-Sequence Turn-Off Stars from the SDSS/SEGUE Sample

TL;DR

This study analyzes the chemical compositions of seven CEMP turn-off stars from SDSS/SEGUE, revealing insights into their nucleosynthesis history, binary interactions, and kinematic properties, with implications for understanding early Galactic chemical evolution.

Contribution

It provides detailed abundance measurements of CEMP turn-off stars, highlighting the preservation of carbon-enhanced material and variability in s-process efficiency in metal-poor AGB stars.

Findings

CEMP turn-off stars have higher [C/H] than giants with less dispersion.

Most SDSS/SEGUE CEMP stars are associated with the outer halo.

Significant variation in s-process element enrichment among the stars.

Abstract

The chemical compositions of seven Carbon-Enhanced Metal-Poor (CEMP) turn-off stars are determined from high-resolution spectroscopy. Five of them are selected from the SDSS/SEGUE sample of metal-poor stars. The effective temperatures of these objects are all higher than 6000 K, while their metallicities, parametrized by [Fe/H], are all below -2. Six of our program objects exhibit high abundance ratios of barium ([Ba/H]> +1), suggesting large contributions of the products of former AGB companions via mass transfer across binary systems. Combining our results with previous studies provides a total of 20 CEMP main-sequence turn-off stars for which the abundances of carbon and at least some neutron-capture elements are determined. Inspection of the [C/H] ratios for this sample of CEMP turn-off stars show that they are generally higher than those of CEMP giants; their dispersion in this ratio is also smaller. We take these results to indicate that the carbon-enhanced material provided from the companion AGB star is preserved at the surface of turn-off stars with no significant dilution. In contrast, a large dispersion in the observed [Ba/H] is found for the sample of CEMP turn-off 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 SDSS/SEGUE sample exhibit kinematics that are associated with membership in the outer-halo population, a remarkably high fraction.