Binarity among CEMP-no stars: an indication of multiple formation pathways?^⋆,⋆⋆

¹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
e-mail: aarentsen@aip.de
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

Received: 28 August 2018
Accepted: 4 November 2018

Abstract

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.