APOGEE [C/N] Abundances Across the Galaxy: Migration and Infall from Red
  Giant Ages
  We present [C/N]-[Fe/H] abundance trends from the SDSS-IV Apache Point
Observatory Galactic Evolution Experiment (APOGEE) survey, Data Release 14
(DR14), for red giant branch stars across the Milky Way Galaxy (MW, 3 kpc $<$ R
$<$ 15 kpc). The carbon-to-nitrogen ratio (often expressed as [C/N]) can
indicate the mass of a red giant star, from which an age can be inferred. Using
masses and ages derived by Martig et al., we demonstrate that we are able to
interpret the DR14 [C/N]-[Fe/H] abundance distributions as trends in age-[Fe/H]
space. Our results show that an anti-correlation between age and metallicity,
which is predicted by simple chemical evolution models, is not present at any
Galactic zone. Stars far from the plane ($|$Z$|$ $>$ 1 kpc) exhibit a radial
gradient in [C/N] ($\sim$ $-$0.04 dex/kpc). The [C/N] dispersion increases
toward the plane ($\sigma_{[C/N]}$ = 0.13 at $|$Z$|$ $>$ 1 kpc to
$\sigma_{[C/N]}$ = 0.18 dex at $|$Z$|$ $<$ 0.5 kpc). We measure a disk
metallicity gradient for the youngest stars (age $<$ 2.5 Gyr) of $-$0.060
dex/kpc from 6 kpc to 12 kpc, which is in agreement with the gradient found
using young CoRoGEE stars by Anders et al. Older stars exhibit a flatter
gradient ($-$0.016 dex/kpc), which is predicted by simulations in which stars
migrate from their birth radii. We also find that radial migration is a
plausible explanation for the observed upturn of the [C/N]-[Fe/H] abundance
trends in the outer Galaxy, where the metal-rich stars are relatively enhanced
in [C/N].
