How did Carbon-Enhanced Metal-Poor Stars form?

Abstract

Among the very metal-poor stars observed in the Galactic halo there is a large fraction of stars which show a great enhancement in the abundance of carbon and other heavy elements. These stars are called CEMP (carbon-enhanced metal-poor) stars. One explanation for these observed enhancements is that in the past CEMP stars have accreted mass from an AGB binary companion. This scenario is supported by the evidence that CEMP stars are mostly found in spectroscopic binary systems. Different aspects still need to be clarified: (1) the efficiency of the mass-transfer process and its effects on the dynamics of the system; (2) the consequences of this mass-transfer for the overall population of metal poor stars and (3) its implications for the initial mass function at early epochs. Recent hydrodynamical simulations suggest a new model for mass-transfer, intermediate between wind accretion and RLOF and applicable in the case of slow and dense AGB winds, which under specific conditions can lead to an accretion efficiency much higher than the classical predictions. We present preliminary results of population simulations in which we explore this new model and its consequences for the CEMP stars. First results suggest a doubling of the CEMP formation rate.