Is Thermohaline Mixing the Full Story? Evidence for Separate Mixing Events near the Red Giant Branch Bump

TL;DR

This paper examines whether thermohaline mixing alone can explain the observed separate chemical depletion events near the red giant branch bump, finding it insufficient and suggesting the need for more complex models.

Contribution

The study provides observational evidence that challenges the sufficiency of thermohaline mixing to explain chemical abundance changes in red giants.

Findings

Lithium decreases before [C/N], contrary to thermohaline predictions.

Simulations show simultaneous drops in abundances, conflicting with observations.

Thermohaline mixing alone cannot account for the separate depletion events.

Abstract

The abundances of mixing--sensitive elements including lithium, [C/N], and 12C/13C are known to change near the red giant branch bump. The explanation most often offered for these alterations is double diffusive thermohaline mixing in the stellar interior. In this analysis, we investigate the ability of thermohaline mixing to explain the observed timing of these chemical depletion events. Recent observational measurements of lithium and [C/N] show that the abundance of lithium decreases before the abundance of [C/N], whereas numerical simulations of the propagation of the thermohaline mixing region computed with MESA show that the synthetic abundances drop simultaneously. We therefore conclude that thermohaline mixing alone cannot explain the distinct events of lithium depletion and [C/N] depletion, as the simultaneity predicted by simulations is not consistent with the observation of separate drops. We thus invite more sophisticated theoretical explanations for the observed temporal separation of these chemical depletion episodes as well as more extensive observational explorations across a range of masses and metallicities.