  Carbon-enhanced metal-poor stars (CEMP) are observed as a substantial
fraction of the very metal-poor stars in the Galactic halo. Most CEMP stars are
also enriched in s-process elements and these are often found in binary
systems. This suggests that the carbon enrichment is due to mass transfer in
the past from an asymptotic giant branch (AGB) star on to a low-mass companion.
Models of binary population synthesis are not able to reproduce the observed
fraction of CEMP stars without invoking non-standard nucleosynthesis or a
substantial change in the initial mass function. This is interpreted as
evidence of missing physical ingredients in the models. Recent hydrodynamical
simulations show that efficient wind mass transfer is possible in the case of
the slow and dense winds typical of AGB stars through a mechanism called wind
Roche-lobe overflow (WRLOF), which lies in between the canonical
Bondi-Hoyle-Lyttleton (BHL) accretion and Roche-lobe overflow. WRLOF has an
effect on the accretion efficiency of mass transfer and on the angular momentum
lost by the binary system. The aim of this work is to understand the overall
effect of WRLOF on the population of CEMP stars. To simulate populations of
low-metallicity binaries we combined a synthetic nucleosynthesis model with a
binary population synthesis code. In this code we implemented the WRLOF
mechanism. We used the results of hydrodynamical simulations to model the
effect of WRLOF on the accretion efficiency and we took the effect on the
angular momentum loss into account by assuming a simple prescription. As a
result the number of CEMP stars predicted by our model increases by a factor
1.2-1.8 compared to earlier results that consider the BHL prescription.
Moreover, higher enrichments of carbon are produced and the final orbital
period distribution is shifted towards shorter periods.

Solar and Stellar Astrophysics