Unifying Transport Models of Thermohaline Convection in Stars

TL;DR

This paper refines the theoretical understanding of thermohaline convection in stars by analyzing unstable modes and their impact on turbulent diffusion coefficients, aiming to resolve existing discrepancies in stellar mixing models.

Contribution

It completes the linear stability analysis, identifies different unstable modes, and proposes a self-consistent way to interpolate between theoretical scalings of the diffusion coefficient.

Findings

Identified two types of unstable modes: slow large-scale and fast small-scale.

Reevaluated the turbulent diffusion coefficient considering the full spectrum of modes.

Proposed that future simulations can determine the dominant modes and resolve efficiency uncertainties.

Abstract

Thermohaline convection is a standard chemical mixing process in stellar interiors, yet its mixing efficiency is not fully settled. Competing theories predict turbulent diffusion coefficients, $D_\mu$, that can differ by orders of magnitude, leading to uncertainties in stellar models and interpretations of observations. This paper explores a potential resolution to existing discrepancies. We first complete the linear stability theory and identify two types of unstable modes: slow growing modes at large length scales and fast growing modes at small length scales. We then reevaluate $D_\mu$ considering the full spectrum of unstable modes and find that it can self-consistently interpolate between previously proposed theoretical scalings across the instability parameter space. The question of thermohaline mixing efficiency in stars may be settled by future simulations that quantify the scale-dependent contributions of fast and slow modes to $D_\mu$ and determine how the modes dominating the transport change across parameter space.