The origins of two classes of carbon-enhanced, metal-poor stars

We have compiled composition, luminosity, and binarity information for carbon-enhanced, metal-poor ( CEMP) stars reported by recent studies. We divided the CEMP star sample into two classes having high and low abundances, respectively, of the s-process elements and consider the abundances of several isotopes, in particular, C-12, C-13, and N-14, as well as the likely evolutionary stages of each star. Despite the fact that objects in both groups were selected from the same surveys (primarily the HK survey), without a priori knowledge of their s-process element abundances, we identify the following remarkable differences between the two classes: s-element-rich CEMP (CEMP-s) stars occupy a wide range of evolutionary states, but do not have a strongly evolved C-13/N-14 ratio, whereas s-element normal CEMP stars (CEMP-no) are found only high up the first-ascent giant branch and possess C-13/N-14 ratios approaching the CN cycle equilibrium value. We argue that these observational constraints can be accommodated by the following scenarios. CEMP-s stars acquire their distinctive surface compositions during their lifetimes when mass is transferred from an AGB companion that has recently synthesized C-12 and s-process elements. Such mass-accreting stars can be enriched at almost any stage of their evolution and hence are found throughout the H-R diagram. Dilution of transferred surface material as the accretor ascends the giant branch and its surface convective zone deepens may reduce the number of such stars, whose surfaces remain C-rich at high luminosities. Many, but not necessarily all, such stars should currently be in binary systems. Li-preserving CEMP-s stars may require a different explanation. In contrast, a CEMP-no star is proposed to have formed from gas that was enriched in C-12 from the triple-alpha process in a previous generation of stars, some of which has been converted to C-13 and N-14 during the present star's giant branch evolution. The binary fraction of such stars should be the same as that of non-carbon-enhanced, metal-poor stars.