Vol. 586
In section 7. Stellar structure and evolution

Binary properties of CH and carbon-enhanced metal-poor stars

by A. Jorissen, S. Van Eck, H. Van Winckel, et al. A&A 586, A158

Large-scale surveys to search for metal-poor stars in the Galaxy have resulted in the discovery of a large number of carbon-enhanced metal-poor stars (CEMP stars). The origin of abundance anomalies in these stars has been the subject of numerous recent investigations. In the standard picture, the carbon excess is produced by an asymptotic giant branch (AGB) star that was once the primary star in a binary system. Carbon and other s-process elements are then dredged up and subsequently transferred onto the companion. Interestingly, the CH stars—so-called because they exhibit strong features due to the CH molecule—otherwise appear to be spectroscopically indistinguishable from the CEMP-s stars. The aim of the study presented by Jorissen et al. was to compare the orbital properties, such as the period and eccentricity of CEMP-s and CH stars, as well as to analyze their mass functions. This provides information on when and how the mass transfer from the AGB star is likely to have occurred. They found no significant difference between the CH and CEMP-s systems. Furthermore, the companion masses were found to peak in the 0.5-0.7 Msun range, providing further support for the idea that the former primary was an AGB star that has evolved into a white dwarf companion of the star that is currently observable. Thus, the attribution of different names to the CH and CEMP-s groups of stars appears to have arisen only as a result of different discovery channels.