Evolution of carbon-enhanced metal-poor stars originating from mass transfer in stellar binaries

Abstract

As the hydrogen and helium created shortly after the Big Bang is gradually processed into heavier elements by successive generations of stars, the overall metal content of the Universe increases. Metal-poor stars which, owing to their low mass, evolve slowly and have ages comparable to the age of the Universe are thus relics of the earliest stellar generations, and recent decades have seen dedicated efforts to discover these stars in ever greater numbers. An unexpected outcome of these efforts is the discovery that a large fraction of these stars are rich in carbon compared to the Sun.<br /> These carbon-enhanced metal-poor (CEMP) stars are a chemically diverse population, and this work concerns the evolution of CEMP stars rich in elements produced by slow neutron capture nucleosynthesis (CEMP-s stars), which are believed to have originated from accreting chemically enriched material from a binary companion in a thermally pulsing asymptotic giant branch (AGB) stage. As such, CEMP-s stars are commonly used to test models of metal-poor AGB stars, an important contributor to the chemical evolution of the Universe.<br /> The evolution of CEMP-s stars is modelled from the zero-age main sequence through the accretion phase and up to the end of the red giant branch with an updated version of the STARS stellar evolution code. Particular attention is paid to the evolution of the surface chemical composition of CEMP-s stars, which is shown to be modified following mass transfer as a result of the competition between various mixing processes taking place within these stars. The mixing processes considered here in detail are thermohaline convection, atomic diffusion, and rotational mixing. Thermohaline convection dilutes the transferred material by rapidly (compared to the main sequence lifetime) mixing it with the material originally present in the star. As a result, the accreted material is diluted by a factor from two to more than ten depending mostly on how much mass is accreted. Atomic diffusion, and radiative forces in particular, accelerate different chemical elements discriminately and tend to make the surface layers of CEMP-s stars poor in carbon but rich in other metals like iron. This is at odds with observations and suggests that some other process is actively counteracting atomic diffusion. It is then demonstrated that this other process could be the turbulent mixing arising in rotating CEMP-s stars. At surface rotation velocities consistent with those observed, atomic diffusion is found to have little effect on the chemical composition of the surface of CEMP-s stars as a result.<br /> It is also found that the amount of mass that can be accreted by the progenitors of CEMP-s and related stars may be limited by the angular momentum content of the transferred material. In particular, explaining the most chemically enriched stars, which have likely accreted the most mass, may require that the material loses most of its angular momentum during the accretion. The rotation velocities observed in CEMP-s stars are also suggestive of angular momentum loss, either during the accretion or the evolution following mass transfer.