Carbon-Enhanced Metal-Poor Stars and the Need for an Intermediate Neutron Capture Process

Carbon-enhanced metal-poor (CEMP) stars in the Galactic Halo display enrichments in heavy elements associated with either the s (slow) or the r (rapid) neutron-capture process (e.g., barium and europium respectively), and in some cases they display evidence of both. The abundance patterns of these CEMP-s/r stars, which show both Ba and Eu enrichment, are particularly puzzling since the s and the r processes require neutron densities that are more than ten orders of magnitude apart, and hence are thought to occur in very different stellar sites. We investigate whether the abundance patterns of CEMP-s/r stars can arise from the nucleosynthesis of the intermediate neutron-capture process (the i process), which is characterised by neutron densities between those of the s and the r processes. Using nuclear network calculations, we study neutron capture nucleosynthesis at different constant neutron densities n ranging from 10(7) to 10(15) cm(-3). Neutron densities on the highest side of this range result in abundance patterns that show an increased production of heavy s- and r-process elements but similar levels of the light s-process elements. With our i-process model, we are able to reproduce the abundance patterns of 20 CEMP-s/r stars that could not be explained by s-process nucleosynthesis.