3D hydrodynamics simulations of a 3 $M_{\odot}$ core-helium burning star

TL;DR

This study uses 3D hydrodynamics simulations to explore the internal mixing processes of core-helium burning stars, revealing that overshooting may eliminate semiconvective layers and impacting stellar evolution models.

Contribution

First full-sphere 3D hydrodynamics simulations of CHeB star interiors, analyzing mixing and semiconvection with different initial conditions and resolutions.

Findings

Semiconvective layers dominated by gravity waves show no measurable mixing.

Overshooting from the convective core can homogenize semiconvective interfaces.

Results suggest CHeB stars may lack semiconvection zones altogether.

Abstract

The inner structure of core-helium burning (CHeB) stars remains uncertain due to the yet unknown nature of mixing at the boundary of their cores. Large convective cores beyond a bare Schwarzschild model are favoured both from theoretical arguments and from asteroseismological constraints. However, the exact nature of this extra mixing, and in particular the possible presence of semiconvective layers, is still debated. In this work, we approach this problem through a new avenue by performing the first full-sphere 3D hydrodynamics simulations of the interiors of CHeB stars. We use the PPMstar explicit gas dynamics code to simulate the inner 0.45 $M_{\odot}$ of a 3 $M_{\odot}$ CHeB star. Simulations are performed using different Cartesian grid resolutions (768$^3$, 1152$^3$ and 1728$^3$) and heating rates. We use two different initial states, one based on MESA's predictive mixing scheme (which significantly extends the core beyond the Schwarzschild boundary) and one based on the convective premixing approach (which exhibits a semiconvective interface). The general behaviour of the flow in the convective core and in the stable envelope (where internal gravity waves are observed) is consistent with our recent simulations of core convection in massive main-sequence stars, and so are the various luminosity scaling relations. The semiconvective layers are dominated by strong internal gravity waves that do not produce measurable species mixing, but overshooting motions from the convective core gradually homogenize the semiconvective interface. This process can possibly completely erase the semiconvective layers, which would imply that CHeB stars do not harbour a semiconvection zone.