The binary frequency among carbon-enhanced, s- process-rich, metal-poor stars

We discuss radial velocities for a sample of carbon-enhanced, s-process-rich, very metal-poor (CEMP-s) stars, analyzed with high-resolution spectroscopy obtained over multiple epochs. We find that similar to 68% of the stars in the sample show evidence of radial velocity variations. The expected detection fraction for these stars, adopting the measured binary fraction in the field (similar to 60%) and assuming that they share the same period and eccentricity distribution, is similar to 22%. Even if one assumes that the true binary fraction of these stars is 100%, the expected detection percentage is similar to 36%. These values indicate that the binary fraction among CEMP-s stars is higher than the field binary fraction, suggesting that all of these objects are in double ( or multiple) systems. The fact that the observed frequency of velocity variation exceeds the expected detection fraction in the case of an assumed binary fraction of 100% is likely due to a more restricted distribution of orbital periods for these objects, as compared to normal field binaries. Our results indicate that CEMP-s stars are the metal-poor analogs of classical CH stars.