Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
2 McDonald Observatory, University of Texas at Austin, HC75 Box 1337-MCD, Fort Davis, TX 79734, USA
3 Department of Physics & Astronomy, University of Victoria, Victoria, BC V8W 3P2, Canada
4 South African Astronomical Observatory (SAAO), Observatory Road Observatory Cape Town, WC 7925, South Africa
5 NRC Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
Accepted: 4 November 2018
Carbon-enhanced metal-poor (CEMP) stars comprise a high percentage of stars at the lowest metallicities. The stars in the CEMP-no subcategory do not show any s-process enhancement and therefore cannot easily be explained by transfer of carbon and s-process elements from a binary AGB companion. We have performed radial velocity monitoring of a sample of 22 CEMP-no stars to further study the role that binarity plays in this type of CEMP star. We find four new binary CEMP-no stars based on their radial velocity variations; this significantly enlarges the population of known binaries to a total of 11. One of the new stars found to be in a binary system is HE 0107–5240, which is one of the most iron-poor stars known. This supports the binary transfer model for the origin of the abundance pattern of this star. We find a difference in binary fraction in our sample that depends on the absolute carbon abundance, with a binary fraction of 47 +15−14% for stars with a higher absolute carbon abundance and 18 +14−9% for stars with a lower absolute carbon abundance. This might imply a relation between a high carbon abundance and the binarity of a metal-poor star. Although binarity does not equate to mass transfer, there is a possibility that a CEMP-no star in a binary system has been polluted, and care has to be taken in the interpretation of their abundance patterns. We furthermore demonstrate the potential of Gaia of discovering additional binary candidates.
Key words: stars: chemically peculiar / binaries: spectroscopic / stars: AGB and post-AGB / Galaxy: halo / galaxies: formation
Based on observations made with the Southern African Large Telescope (SALT) and the Canada-France-Hawaii Telescope (CFHT).
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