Carbon-enhanced metal-poor stars: effects of binary evolution at low metallicity

Abstract

Recent spectroscopic surveys have revealed a large number of extremely metal-poor stars in the Galactic halo. Many of these stars are being subjected to detailed spectroscopic analysis, and a surprisingly large fraction, about 25 %, turn out to be carbon-rich stars with enhancements of C by as much as a factor 100. The majority of these carbon-enhanced metal-poor (CEMP) stars also show enhancements of heavy s-process elements, in particular of lead. Many of these stars have been found to be spectroscopic binary systems. Their binarity and abundance patterns strongly suggest that most, possibly all, CEMP stars have been polluted by a companion star while in an advanced stage of evolution, which has long since faded away. They provide strong indications that (1) binary stars were as common among very low-metallicity populations as they are in the Galactic disk and (2) binaries may have played an important role in shaping the abundance patterns of the earliest generations of stars.