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

We present models for the slow neutron-capture process (s process) in asymptotic giant branch (AGB) stars of metallicity [Fe/H]=-2.3 and masses 0.9 Msun to 6 Msun. We encountered different regimes of neutron-capture nucleosynthesis increasing in importance as the stellar mass decreases: the 22Ne(alpha,n)25Mg reaction activated during the thermal pulses, the 13C(alpha,n)16O reaction activated in radiative conditions during the interpulse periods, and the 13C(alpha,n)16O reaction activated during the thermal pulses, also as a result of mild proton ingestion episodes. The models where the 13C burns radiatively (masses ~ 2 Msun) produce an overall good match to 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 our models can provide a match to the composition of CEMP stars also showing rapid-process enhancements (CEMP-s/r). The models fail to reproduce the observed Eu abundances, and they also fail to reproduce the correlation between the Eu and Ba abundances. They also cannot match the ratio of heavy to light s-process elements observed in many CEMP-s/r stars, which can be more than ten times higher than in the solar system. To explain the composition of CEMP-s/r stars we need to invoke the existence of a different "s/r" neutron-capture process either with features in-between the s and the r processes, or generated by superpositions of different neutron-capture processes in the same astrophysical site or in sites linked to each other - for example, in multiple stellar systems.