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

Abstract

The HERMES spectrograph installed on the 1.2-m Mercator telescope has been used to monitor the radial velocity of 13 lowmetallicity 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 lowmass 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 highermetallicity 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.

The role of binaries in the enrichment of the early Galactic halo

“…-HE 0111−1346 and HE 0507−1653 were also observed extensively by Jorissen et al (2016b), with results agreeing with ours within the errors. -HE 0430−1609 and HE 1523−1155 have suspiciously low rms errors for their orbital solutions.…”

“…Signatures of similar pulsations in another two CEMP stars are found in the sample of Jorissen et al (2016b), but not among carbon-normal VMP/EMP r-process-enhanced stars. This suggests that the high molecular opacities of C-rich stellar atmospheres may be their source, but photometric confirmation of their existence and probable long periods in field stars is difficult from the ground.…”

“…5 exhibits the expected overall feature (Jorissen et al 1998(Jorissen et al , 2016b) of (near-) circular orbits up to a cut-off that depends on the age of the systems, but can be estimated at ≈200 days for Population I and Population II giants of all ages. This is commonly ascribed to tidal circularisation of the orbits, which is more advanced the older the star; the few moderately eccentric, shorter-period binaries are probably younger and/or more metal-poor stars in uncertain stages of evolution.…”

Precise radial velocities of giant stars

“…However, we broadened the width of the window centered on Hα, from ±15 km s −1 used by Kürster et al (2003) for Barnard's star to ±45.68 km s −1 (±1 Å) It is worth to note, however, that HD 59686 A shows some similarities to carbon-enhanced metal-poor (CEMP) stars (see Beers & Christlieb 2005;Masseron et al 2010;Placco et al 2014) in the sense that these are evolved giants, they reside in binary systems, and the secondary is very likely a white dwarf (provided that HD 59686 B is confirmed to be a white dwarf). Recently, Jorissen et al (2016a) has identified low-amplitude RV variations superimposed on the binary trend in 3 CEMP stars in a sample of 13. They show periods of about one year and RV semi-amplitudes of hundreds of m/s.…”

The Binary Nature of Ch-Like Stars

“…It is worth noting that neither pre-mass-transfer systems nor post-mass-transfer systems populate the avoidance zone in the bottom right corner of the (e-log P) plane (Mathieu 1994). The two exceptions, with P ∼ 3000-4000 days and 0.00 < e < 0.05, are due to the low accuracies of the eccentricities; high accuracies are difficult to obtain for such long-period systems, as discussed by Jorissen et al (2016).…”