Carbon-Enhanced Metal-Poor Stars and the s/r Process

We have performed calculations for the slow neutron capture process (the s process) in stellar models of mass from 0.9 M-circle dot to 6 M-circle dot at a metallicity of [Fe/H]=-2.3. The models were computed with two different stellar evolution codes and our results compared to the composition of the elements heavier than Fe observed in carbon-enhanced metal-poor (CEMP) stars. Our models provide an explanation for the compositions of CEMP stars showing s-process signature (CEMP-s) but not for the composition of CEMP stars showing both s-process and rapid neutron-capture (r-process) signature (CEMP-s/r). To explain the composition of CEMP-s/r we propose the existence of a "s/r" neutron-capture process, which may be a superposition of, or a process in-between, the classical s and r processes. Here we attempt at understanding some qualitative features of the s/r process on the basis of the observational constraints.