  Carbon abundances in first-ascent giant stars are usually lower than those of
their main-sequence counterparts. At moderate metallicities, stellar evolution
of single stars cannot account for the existence of red-giant branch stars with
enhanced carbon abundances. The phenomenon is usually interpreted as resulting
from past mass transfer from an evolved binary companion now in the white dwarf
evolutionary stage. Aims: We aim to confirm the links between [C/O]
enhancement, s-process element enhancement and binary fraction using
large-scale catalogues of stellar abundances and probable binary stars.
Methods: We use a large data set from the 17 data release of the
SDSS-IV/APOGEE~2 survey to identify carbon-enhanced stars in the Galactic disk.
We identify a continuum of carbon enrichment throughout three different
sub-populations of disk stars and explore links between the degree of carbon
enrichment and binary frequency, metallicity and chemical compositions.
Results: We verify a clear correlation between binary frequency and enhancement
in the abundances of both carbon and cerium, lending support to the scenario
whereby carbon-enhanced stars are the result of mass transfer by an evolved
binary companion. In addition, we identify clustering in the carbon abundances
of high-$\alpha$ disk stars, suggesting that those on the high metallicity end
are likely younger, in agreement with theoretical predictions for the presence
of a starburst population following the gas-rich merger of the
Gaia-Enceladus/Sausage system.
