Turbulent Mixing and Nuclear Burning in Stellar Interiors

TL;DR

This paper investigates turbulent mixing and nuclear burning in stellar interiors, specifically the neon-oxygen shell merger in a 23 solar mass star, revealing a new quasi-steady state and developing turbulence equations for better interpretation.

Contribution

It introduces a mean-field turbulence framework to analyze complex stellar mixing and burning processes, highlighting differences from traditional 1D models and providing tools for sub-grid-scale effect quantification.

Findings

Discovery of a new quasi-steady state in shell merging

Extended neon burning layer within the convection zone

Validation of Kolmogorov turbulence dissipation rate

Abstract

The turbulent burning of nuclei is a common phenomenon in the evolution of stars. Here we examine a challenging case: the merging of the neon and oxygen burning shells in a 23 M$_{\odot}$ star. A previously unknown quasi-steady state is established by the interplay between mixing, turbulent transport, and nuclear burning. The resulting stellar structure has two burning shells within a single convection zone. We find that the new neon burning layer covers an extended region of the convection zone, with the burning peak occurring substantially below where the Damk\"ohler number first becomes equal to unity. These characteristics differ from those predicted by 1D stellar evolution models of similar ingestion events. We develop the mean-field turbulence equations that govern compositional evolution, and use them to interpret our data set. An important byproduct is a means to quantify sub-grid-scale effects intrinsic to the numerical hydrodynamic scheme. For implicit large eddy simulations, the analysis method is particularly powerful because it can reveal where and how simulated flows are modified by resolution, and provide straightforward physical interpretations of the effects of dissipation or induced transport. Focusing on the mean-field composition variance equations for our analysis, we recover a Kolmogorov rate of turbulent dissipation without it being imposed, in agreement with previous results which used the turbulent kinetic energy equation.