[C/N] in red giants and across the Milky Way
February 5th, 2019
Despite being some of the most abundant elements in the Universe the determination and understanding of the chemical evolution of C and N is still very uncertain. One of the main limitations in understanding chemical evolution of C and N is the fact that C and N are altered as during the first dredge-up on the red giant branch. We present old red giants at various metallicities and luminosities in a sample that is more than 100 times larger than the seminal work of Gratton et al. 2000. Using this we can see the impact of the first dredge-up as well as the on set of "extra" mixing at the bump in the luminosity function for giants more metal-poor than [Fe/H] < -0.4. These observations can be used to constrain future models of mixing. At a fixed metallicity younger stars have a stronger mixing response during dredge-up. This fact allows up to infer ages from the first dredge-up [C/N] ratio. 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]. The [C/N] dispersion increases toward the plane. We measure a disk metallicity gradient for the youngest stars from 6 kpc to 12 kpc, which is in agreement with the gradient found from other surveys. Older stars exhibit a flatter gradient, 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].