The elusive origin of Carbon-Enhanced Metal-Poor stars

Abstract

Very metal-poor stars of the halo are the relics of the star formation in the early Galaxy. Among these stars carbon-enhanced metal-poor (CEMP) stars count for 9-25%. Most CEMP stars are also enriched in s-process elements and are called CEMP-s stars. The large number of CEMP-s stars observed in binary systems suggests that the chemical enrichment is due to wind mass transfer in the past from an AGB donor star on to a low-mass companion, the star that we are observing today. However, binary population synthesis models predict CEMP fractions of only ≈ 2%. As an alternative to the canonical Bondi-Hoyle-Lyttleton (BHL) wind accretion model, recent hydrodynamical simulations suggest an efficient mode of wind mass transfer, called wind Roche-lobe overflow (WRLOF), can reproduce observations of AGB winds in binary systems. We use our population synthesis model to test the consequences of WRLOF on a population of CEMP stars. Compared to previous predictions based on the BHL model we find a modest increase of the fraction of CEMP stars and substantial differences in the distributions of carbon and periods in the population of CEMP stars. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence.