  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 with very different physical conditions. 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$^{-3}$. With respect to the classical s process resulting from neutron
densities on the lowest side of this range, neutron densities on the highest
side result in abundance patterns that show an increased production of heavy
s-process and r-process elements but similar abundances of the light s-process
elements. Such high values of n may occur in the thermal pulses of asymptotic
giant branch (AGB) stars due to proton ingestion episodes. Comparison to the
surface abundances of 20 CEMP-s/r stars show that our modelled i-process
abundances successfully reproduce observed abundance patterns that could not be
previously explained by s-process nucleosynthesis. Because the i-process models
fit the abundances of CEMP-s/r stars so well, we propose that this class should
be renamed as CEMP-i.
