| Very metal-poor stars in the Galactic halo are the only remaining relics from the primordial conditions under which the Milky Way formed. Due to their low masses these stars have hardly evolved since they were formed. A large fraction of these stars are surprisingly enriched in carbon and other heavy elements. Their chemical enrichment is considered to result from mass transfer from a more massive binary companion during its asymptotic giant branch phase. Indeed, these carbon-enhanced metal-poor stars are generally found in spectroscopic binary systems. It has been argued that their large number, about 20 per cent of all metal-poor stars, constitutes evidence for a modified initial mass function at early epochs, containing a much higher proportion of intermediate-mass stars. In order to draw reliable conclusions about the early evolution of our Milky Way from its metal-poor halo population, the formation and evolution of carbon-enhanced metal-poor stars needs to be well understood. This is the main goal of this project. Recent studies indicate that the surface abundances of CEMP stars do not represent the nucleosynthesis output of their companions, but have undergone substantial changes during their subsequent evolution. In this project we request one PhD position to answer the following outstanding questions: (1) how do the abundances of low-mass metal-poor stars evolve after accreting nuclear-processed material from an evolved asymptotic giant star and (2) what are the consequences of this mass transfer for the overall population of CEMP stars and its implications for the initial mass function at early epochs. |
