Vol. 586
In section 7. Stellar structure and evolution

The role of binaries in the enrichment of the early Galactic halo. II. Carbon-enhanced metal-poor stars: CEMP-no stars

by T.T. Hansen, J. Andersen, B. Nordström, T. C. Beers, V.M. Placco, J. Yoon, and L.A. Buchhave A&A 586, A160


The chemical composition of most Milky Way halo stars is typically very uniform. However, a portion of the population, which increases with decreasing metallicity, exhibit dramatic enhancements in their abundances of carbon: the so-called carbon-enhanced metal-poor (CEMP) stars. A key question is whether this reflects the properties of the gas out of which these stars were born or if it is due to local, post-birth mass transfer of chemically processed material from a binary companion. If the latter is the case, then CEMP stars should all be in binary systems. This paper presents detailed monitoring of 24 CEMP stars that show no enhancement of neutron-capture elements (CEMP-no stars). High spectral resolution, low signal-to-noise spectra were obtained monthly over a period of 8 years, using the FIES spectrograph at the Nordic Optical Telescope on La Palma. The result is that most stars in this sample show no evidence of radial-velocity variations and are thus consistent with being single stars. This result allows the authors to conclude that the nucleosynthetic process responsible for the carbon excess in these ancient CEMP-no stars is not likely to be due to mass transfer binaries, but the carbon must have been present in the early Galactic interstellar medium, coming from an even earlier, external source, which strongly indicates that the CEMP-no stars are probably bona fide second-generation stars.