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Standard stellar evolution theory poorly predicts the surface abundances ofchemical species in low-mass, red giant branch (RGB) stars. Observations showan enhancement of p-p chain and CNO cycle products in red giant envelopes,which suggests the existence of non-canonical mixing that brings interiorburning products to the surface of these stars. The 12C/13C ratio is a highlysensitive abundance metric used to probe this mixing. We investigate extra RGBmixing by examining (1) how 12C/13C is altered along the RGB and (2) how12C/13C changes for stars of varying age and mass. Our sample consists of 43red giants spread over 15 open clusters from the Sloan Digital Sky Survey'sAPOGEE DR17 that have reliable 12C/13C ratios derived from their APOGEEspectra. We vetted these 12C/13C ratios and compared them as a function ofevolution and age/mass to the standard mixing model of stellar evolution and toa model that includes prescriptions for RGB thermohaline mixing and stellarrotation. We find that the observations deviate from standard mixing models,implying the need for extra mixing. Additionally, some of the abundancepatterns depart from the thermohaline model, and it is unclear whether thesedifferences are due to incomplete observations, issues inherent to the model,our assumption of the cause of extra mixing, or any combination of thesefactors. Nevertheless, the surface abundances across our age/mass range clearlydeviate from the standard model, agreeing with the notion of a universalmechanism for RGB extra mixing in low-mass stars.

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