Many observed CEMP stars are found in binary systems and show enhanced
abundances of s-elements. The origin of the chemical abundances of these
CEMP-s stars is believed to be accretion in the past of enriched material
from a primary star in the AGB phase. We investigate the mechanism of mass
transfer and the process of nucleosynthesis in low-metallicity AGB stars by
modelling the binary systems in which the observed CEMP-s stars were formed.
For this purpose we compare a sample of 67 CEMP-s stars with a grid of
binary stars generated by our binary evolution and nucleosynthesis model. We
classify our sample CEMP-s stars in three groups based on the observed
abundance of europium. In CEMP−s/r stars the europium-to-iron ratio is more
than ten times higher than in the Sun, whereas it is lower than this threshold
in CEMP−s/nr stars. No measurement of europium is currently available for
CEMP-s/ur stars. On average our models reproduce well the abundances observed
in CEMP-s/nr stars, whereas in CEMP-s/r stars and CEMP-s/ur stars the
abundances of the light-s elements are systematically overpredicted by our
models and in CEMP-s/r stars the abundances of the heavy-s elements are
underestimated. In all stars our modelled abundances of sodium overestimate the
observations. This discrepancy is reduced only in models that underestimate the
abundances of most of the s-elements. Furthermore, the abundance of lead is
underpredicted in most of our model stars. These results point to the
limitations of our AGB nucleosynthesis model, particularly in the predictions
of the element-to-element ratios. Finally, in our models CEMP-s stars are
typically formed in wide systems with periods above 10000 days, while most of
the observed CEMP-s stars are found in relatively close orbits with periods
below 5000 days.Comment: 23 pages, 8 figures, accepted for publication on Astronomy &
Astrophysic
