Binary properties of CH and carbon-enhanced metal-poor stars^⋆

¹ Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles, Campus Plaine, C.P. 226, Boulevard du Triomphe, 1050 Bruxelles, Belgium
e-mail: ajorisse@astro.ulb.ac.be
² Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
³ ESO, Alonso de Córdova 3107, Casilla 19001, Santiago, Chile
⁴ Dark Cosmology Centre, The Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
⁵ Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark
⁶ Observatoire de Genève, Université de Genève, 1290 Versoix, Suisse
⁷ Institute of Astronomy, Cambridge University, Cambridge CB3 OHA, UK

Received: 18 July 2015
Accepted: 16 October 2015

Abstract

The HERMES spectrograph installed on the 1.2-m Mercator telescope has been used to monitor the radial velocity of 13 low-metallicity carbon stars, among which seven carbon-enhanced metal-poor (CEMP) stars and six CH stars (including HIP 53522, a new member of the family, as revealed by a detailed abundance study). All stars but one show clear evidence for binarity. New orbits are obtained for eight systems. The sample covers an extended range in orbital periods, extending from 3.4 d (for the dwarf carbon star HE 0024-2523) to about 54 yr (for the CH star HD 26, the longest known among barium, CH, and extrinsic S stars). Three systems exhibit low-amplitude velocity variations with periods close to 1 yr superimposed on a long-term trend. In the absence of an accurate photometric monitoring of these systems, it is not clear yet whether these variations are the signature of a very low-mass companion or of regular envelope pulsations. The period – eccentricity (P − e) diagram for the 40 low-metallicity carbon stars with orbits now available shows no difference between CH and CEMP-s stars (the latter corresponding to those CEMP stars enriched in s-process elements, as are CH stars). We suggest that they must be considered as one and the same family and that their different names only stem from historical reasons. Indeed, these two families have as well very similar mass-function distributions, corresponding to companions with masses in the range 0.5–0.7 M_⊙, indicative of white-dwarf companions, adopting 0.8–0.9 M_⊙ for the primary component. This result confirms that CH and CEMP-s stars obey the same mass-transfer scenario as their higher-metallicity analogues, barium stars. The P − e diagrams of barium, CH, and CEMP-s stars are indeed very similar. They reveal two different groups of systems: one with short orbital periods (P< 1000 d) and mostly circular or almost circular orbits, and another with longer period and eccentric (e> 0.1) orbits. These two groups either trace different evolutionary channels during the mass-transfer episode responsible for the chemical peculiarities of the Ba/CH/CEMP-s stars, or result from the operation of tidal circularisation in a more recent past, when the current giant star was ascending the first giant branch.