Observational Constraints for Thermohaline Mixing

TL;DR

This paper reviews thermohaline mixing as an extra mixing process in red giant stars, compares models with observations, and highlights challenges in matching carbon depletion data in globular clusters.

Contribution

It analyzes thermohaline mixing in RGB stars, using observations to constrain the formalism and revealing discrepancies between models and cluster data.

Findings

Models match carbon depletion in metal-poor field giants.

Models fail to reproduce carbon depletion in globular cluster giants.

Evidence suggests pre-dredge-up mixing in some cluster giants.

Abstract

We provide a brief review of thermohaline physics and why it is a candidate extra mixing mechanism during the red giant branch (RGB). We discuss how thermohaline mixing (also called $\delta$ $\mu$ mixing) during the RGB due to helium-3 burning, is more complicated than the operation of thermohaline mixing in other stellar contexts (such as following accretion from a binary companion). We try to use observations of carbon depletion in globular clusters to help constrain the formalism and the diffusion coefficient or mixing velocity that should be used in stellar models. We are able to match the spread of carbon depletion for metal poor field giants but are unable to do so for cluster giants, which may show evidence of mixing prior to even the first dredge-up event.