Constraining Metallicity-dependent Mixing and Extra Mixing using [C/N] in Alpha-Rich Field Giants

Abstract

Internal mixing on the giant branch is an important process which affects the evolution of stars and the chemical evolution of the galaxy. While several mechanisms have been proposed to explain this mixing, better empirical constraints are necessary. Here, we use [C/N] abundances in 26097 evolved stars from the SDSS-IV/APOGEE-2 Data Release 14 to trace mixing and extra mixing in old field giants with -1.7< [Fe/H] < 0.1. We show that the APOGEE [C/N] ratios before any dredge-up occurs are metallicity dependent, but that the change in [C/N] at the first dredge-up is metallicity independent for stars above [Fe/H] ~ -1. We identify the position of the red giant branch (RGB) bump as a function of metallicity, note that a metallicity-dependent extra mixing episode takes place for low-metallicity stars ([Fe/H] <-0.4) 0.14 dex in log g above the bump, and confirm that this extra mixing is stronger at low metallicity, reaching $\Delta$ [C/N] = 0.58 dex at [Fe/H] = -1.4. We show evidence for further extra mixing on the upper giant branch, well above the bump, among the stars with [Fe/H] < -1.0. This upper giant branch mixing is stronger in the more metal-poor stars, reaching 0.38 dex in [C/N] for each 1.0 dex in log g. The APOGEE [C/N] ratios for red clump (RC) stars are significantly higher than for stars at the tip of the RGB, suggesting additional mixing processes occur during the helium flash or that unknown abundance zero points for C and N may exist among the red clump RC sample. Finally, because of extra mixing, we note that current empirical calibrations between [C/N] ratios and ages cannot be naively extrapolated for use in low-metallicity stars specifically for those above the bump in the luminosity function.

References in corpus (14)

• The 2.5 m Telescope of the Sloan Digital Sky Survey
• The APOKASC Catalog: An Asteroseismic and Spectroscopic Joint Survey of Targets in the Kepler Fields
• Carbon-Enhanced Metal-Poor Star Frequencies in the Galaxy: Corrections for the Effect of Evolutionary Status on Carbon Abundances
• Target Selection for the SDSS-IV APOGEE-2 Survey
• Young [$\alpha$/Fe]-enhanced stars discovered by CoRoT and APOGEE: What is their origin?
• Can Extra Mixing in RGB and AGB Stars Be Attributed to Magnetic Mechanisms?
• New H-band Stellar Spectral Libraries for the SDSS-III/APOGEE survey
• Chemical Transport and Spontaneous Layer Formation in Fingering Convection in Astrophysics
• Stellar Population Models and Individual Element Abundances I: Sensitivity of Stellar Evolution Models
• APOGEE Chemical Abundances of the Sagittarius Dwarf Galaxy
• Carbon in Red Giants in Globular Clusters and Dwarf Spheroidal Galaxies
• A New Method for the Asteroseismic Determination of the Evolutionary State of Red-Giant Stars
• Nitrogen depletion in field red giants: mixing during the He flash?
• The effects of individual metal contents on isochrones for C, N, O, Na, Mg, Al, Si, and Fe

Cited by in corpus (4)

• APOGEE Data and Spectral Analysis from SDSS Data Release 16: Seven Years of Observations Including First Results from APOGEE-South
• The lowest detected stellar Fe abundance: The halo star SMSS J160540.18-144323.1
• Exploring the stellar age distribution of the Milky Way Bulge using APOGEE
• On Lithium-6 as diagnostic of the lithium-enrichment mechanism in red giants