The Binary Frequency Among Carbon‐enhanced, s ‐Process–rich, Metal‐poor Stars

Abstract

We discuss radial velocities for a sample of carbon-enhanced,s-process-rich, very metal-poor (CEMP-s) stars, analyzed withhigh-resolution spectroscopy obtained over multiple epochs. We find that~68% of the stars in the sample show evidence of radial velocityvariations. The expected detection fraction for these stars, adoptingthe measured binary fraction in the field (~60%) and assuming that theyshare the same period and eccentricity distribution, is ~22%. Even ifone assumes that the true binary fraction of these stars is 100%, theexpected detection percentage is ~36%. These values indicate that thebinary fraction among CEMP-s stars is higher than the field binaryfraction, suggesting that all of these objects are in double (ormultiple) systems. The fact that the observed frequency of velocityvariation exceeds the expected detection fraction in the case of anassumed binary fraction of 100% is likely due to a more restricteddistribution of orbital periods for these objects, as compared to normalfield binaries. Our results indicate that CEMP-s stars are themetal-poor analogs of classical CH stars.Based in part on observations collected at the European SouthernObservatory, Paranal, Chile (ESO Programme 167.D-0173).INAF-Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5,I-35122 Padova, Italy.