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 Mββ of a 3 Mββ CHeB
star. Simulations are performed using different Cartesian grid resolutions
(7683, 11523 and 17283) and heating rates. We use two different
initial states, one based on MESA's predictive mixing scheme (which yields a
large overshoot region) 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 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.Comment: Submitted to MNRAS. Movies available at https://www.ppmstar.or