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We present models for the slow neutron-capture process (s process) inasymptotic giant branch (AGB) stars of metallicity [Fe/H]=-2.3 and masses 0.9Msun to 6 Msun. We encountered different regimes of neutron-capturenucleosynthesis increasing in importance as the stellar mass decreases: the22Ne(alpha,n)25Mg reaction activated during the thermal pulses, the13C(alpha,n)16O reaction activated in radiative conditions during theinterpulse periods, and the 13C(alpha,n)16O reaction activated during thethermal pulses, also as a result of mild proton ingestion episodes. The modelswhere the 13C burns radiatively (masses ~ 2 Msun) produce an overall good matchto carbon-enhanced metal-poor (CEMP) stars showing s-process enhancements(CEMP-s), except they produce too much Na and F. On the other hand, none of ourmodels can provide a match to the composition of CEMP stars also showingrapid-process enhancements (CEMP-s/r). The models fail to reproduce theobserved Eu abundances, and they also fail to reproduce the correlation betweenthe Eu and Ba abundances. They also cannot match the ratio of heavy to lights-process elements observed in many CEMP-s/r stars, which can be more than tentimes higher than in the solar system. To explain the composition of CEMP-s/rstars we need to invoke the existence of a different "s/r" neutron-captureprocess either with features in-between the s and the r processes, or generatedby superpositions of different neutron-capture processes in the sameastrophysical site or in sites linked to each other - for example, in multiplestellar systems.

The s process in asymptotic giant branch stars of low metallicity and the composition of carbon-enhanced metal-poor stars