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

TL;DR

This study uses APOGEE data to investigate non-canonical mixing in red giant stars by analyzing 12C/13C ratios, revealing deviations from standard models and supporting the existence of a universal extra mixing mechanism.

Contribution

It provides the first comprehensive observational analysis of 12C/13C ratios across multiple open clusters, challenging standard stellar evolution models.

Findings

Deviations from standard mixing models observed in 12C/13C ratios.

Evidence supporting a universal mechanism for RGB extra mixing.

Some abundance patterns differ from thermohaline model predictions.

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.