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 survey, Data Release 14 (DR14), for red giant branch stars across the Milky Way (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 anticorrelation between age and metallicity, which is predicted by simple chemical evolution models, is not present at any Galactic zone. Stars far from the plane (vertical bar Z vertical bar > 1 kpc) exhibit a radial gradient in [C/N] (similar to-0.04 dex kpc(-1)). The [C/N] dispersion increases toward the plane (sigma([C/N]) = 0.13 at vertical bar Z vertical bar > 1 kpc to sigma([C/N]) = 0.18 dex at vertical bar Z vertical bar < 0.5 kpc). We measure a disk metallicity gradient for the youngest stars (age < 2.5 Gyr) of -0.060 dex kpc(-1) from 6 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(-1)), 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].