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Context. Degenerate ignition of helium in low-mass stars at the end of the
red giant branch phase leads to dynamic convection in their
helium cores. One-dimensional (1D) stellar modeling of this
intrinsically multi-dimensional dynamic event is likely to be
inadequate. Previous hydrodynamic simulations imply that the
single convection zone in the helium core of metal-rich Pop I
stars grows during the flash on a dynamic timescale. This may
lead to hydrogen injection into the core and to a double convection
zone structure as known from one-dimensional core helium flash
simulations of low-mass Pop III stars.
Aims. We perform hydrodynamic simulations of the core helium flash in
two and three dimensions to better constrain the nature of these
events. To this end we study the hydrodynamics of convection
within the helium cores of a 1.25 M⊙ metal-rich Pop I star
(Z = 0.02), and, for the first time, a 0.85 M⊙ metal-free Pop III star (Z = 0) near the peak of the flash. These models possess single
and double convection zones, respectively.
Methods. We use 1D stellar models of the core helium flash computed with
state-of-the-art stellar evolution codes as initial models for
our multidimensional hydrodynamic study, and simulate the
evolution of these models with the Riemann solver based
hydrodynamics code Herakles, which integrates the Euler equations
coupled with source terms corresponding to gravity and nuclear
burning.
Results. The hydrodynamic simulation of the Pop I model involving a single
convection zone covers 27 h of stellar evolution, while the
hydrodynamic simulations of a double convection zone, in
the Pop III model, span 1.8 h of stellar life. We find
differences between the predictions of mixing length theory and
our hydrodynamic simulations. The simulation of the single
convection zone in the Pop I model shows a strong growth of the
size of the convection zone due to turbulent entrainment. We therefore
predict that for the Pop I model a hydrogen injection phase
(i.e., hydrogen injection into the helium core) will commence after
about 23 days, which should eventually lead to a double
convection zone structure known from 1D stellar modeling of
low-mass Pop III stars. Our two and three-dimensional
hydrodynamic simulations of the double (Pop III) convection zone
model show that the velocity field in the convection zones is
different from that given by stellar evolutionary
calculations. The simulations suggest that the double
convection zone decays quickly, the flow eventually being dominated by
internal gravity waves. The decay could be an artefact caused by the
mapping of the initial stellar model to the numerical grid of our hydrodynamics code
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