An Investigation of Non-Canonical Mixing in Red Giant Stars Using APOGEE 12C/13C Ratios Observed in Open Cluster Stars

Abstract

Standard stellar evolution theory poorly predicts the surface abundances of chemical species in low-mass, red giant branch (RGB) stars. Observations show an enhancement of p-p chain and CNO cycle products in red giant envelopes, which suggests the existence of non-canonical mixing that brings interior burning products to the surface of these stars. The 12C/13C ratio is a highly sensitive abundance metric used to probe this mixing. We investigate extra RGB mixing by examining (1) how 12C/13C is altered along the RGB and (2) how 12C/13C changes for stars of varying age and mass. Our sample consists of 43 red giants spread over 15 open clusters from the Sloan Digital Sky Survey's APOGEE DR17 that have reliable 12C/13C ratios derived from their APOGEE spectra. We vetted these 12C/13C ratios and compared them as a function of evolution and age/mass to the standard mixing model of stellar evolution and to a model that includes prescriptions for RGB thermohaline mixing and stellar rotation. We find that the observations deviate from standard mixing models, implying the need for extra mixing. Additionally, some of the abundance patterns depart from the thermohaline model, and it is unclear whether these differences are due to incomplete observations, issues inherent to the model, our assumption of the cause of extra mixing, or any combination of these factors. Nevertheless, the surface abundances across our age/mass range clearly deviate from the standard model, agreeing with the notion of a universal mechanism for RGB extra mixing in low-mass stars.

13 pages, 6 figures, accepted for publication in MNRAS

References in corpus (21)

• Modules for Experiments in Stellar Astrophysics (MESA)
• A grid of MARCS model atmospheres for late-type stars I. Methods and general properties
• The 2.5 m Telescope of the Sloan Digital Sky Survey
• Painting a portrait of the Galactic disc with its stellar clusters
• APOGEE Data and Spectral Analysis from SDSS Data Release 16: Seven Years of Observations Including First Results from APOGEE-South
• Thermohaline mixing: A physical mechanism governing the photospheric composition of low-mass giants
• The Apache Point Observatory Galactic Evolution Experiment (APOGEE) Spectrographs
• Target Selection for the SDSS-IV APOGEE-2 Survey
• Age determination for 269 $Gaia$ DR2 Open Clusters
• Rotational mixing in low-mass stars II. Self-consistent models of Pop II RGB stars
• The Open Cluster Chemical Abundances and Mapping Survey: IV. Abundances for 128 Open Clusters using SDSS/APOGEE DR16
• The APOGEE Data Release 16 Spectral Line List
• Can Extra Mixing in RGB and AGB Stars Be Attributed to Magnetic Mechanisms?
• Final Targeting Strategy for the SDSS-IV APOGEE-2S Survey
• Final Targeting Strategy for the SDSS-IV APOGEE-2N Survey
• The Open Cluster Chemical Abundances and Mapping Survey: VI. Galactic Chemical Gradient Analysis from APOGEE DR17
• Lithium in red giant stars: Constraining non-standard mixing with large surveys in the Gaia era
• Red clump stars of the Milky Way - laboratories of extra-mixing
• Age and helium content of the open cluster NGC 6791 from multiple eclipsing binary members. III. Constraints from a subgiant
• 12C/13C isotopic ratios in red-giant stars of the open cluster NGC 6791
• Is Thermohaline Mixing the Full Story? Evidence for Separate Mixing Events near the Red Giant Branch Bump

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• Exploring the History of Stellar Mergers with Chemistry: Examining the Origins of Massive $\alpha$-Enriched Stars using Carbon Isotope Ratios