Towards a self-consistent model of the convective core boundary in upper main sequence stars. Part I: 2.5D and 3D simulations

TL;DR

This study uses 2.5D and 3D simulations to investigate convective boundary mixing in main sequence stars, finding a consistent penetration layer that aligns with observational data and improves understanding of stellar evolution.

Contribution

It provides the first detailed 2.5D and 3D simulation analysis of convective boundary penetration in massive stars, linking simulation results with observational constraints.

Findings

Penetration layer thickness converges with grid refinement.

Simulated penetration distances are compatible with observations.

Layer structure varies with luminosity, approaching adiabatic conditions.

Abstract

There is strong observational evidence that the convective cores of intermediate-mass and massive main sequence stars are substantially larger than those predicted by standard stellar-evolution models. However, it is unclear what physical processes cause this phenomenon or how to predict the extent and stratification of stellar convective boundary layers. Convective penetration is a thermal-timescale process that is likely to be particularly relevant during the slow evolution on the main sequence. We use our low-Mach-number Seven-League Hydro code to study this process in 2.5D and 3D geometries. Starting with a chemically homogeneous model of a $15\,\mathrm{M}_\odot$ zero-age main sequence star, we construct a series of simulations with the luminosity increased and opacity decreased by the same factor, ranging from $10^3$ to $10^6$. After reaching thermal equilibrium, all of our models show a clear penetration layer; its thickness becomes statistically constant in time and it is shown to converge upon grid refinement. The penetration layer becomes nearly adiabatic with a steep transition to a radiative stratification in simulations at the lower end of our luminosity range. This structure corresponds to the adiabatic `step overshoot' model often employed in stellar-evolution calculations. The simulations with the highest and lowest luminosity differ by less than a factor of two in the penetration distance. The high computational cost of 3D simulations makes our current 3D data set rather sparse. Depending on how we extrapolate the 3D data to the actual luminosity of the initial stellar model, we obtain penetration distances ranging from $0.09$ to $0.44$ pressure scale heights, which is broadly compatible with observations.