The core helium flash revisited: I. One and two-dimensional hydrodynamic simulations

TL;DR

This study uses advanced 2D and 1D hydrodynamic simulations to clarify the core helium flash's behavior, showing it remains quiescent and influences chemical mixing, contrasting earlier inconclusive or explosive scenarios.

Contribution

It provides detailed multidimensional simulations with microphysics, demonstrating the quiescent nature of the core helium flash and revealing overshooting effects.

Findings

The core helium flash does not disrupt the star.

Convection maintains hydrostatic equilibrium during the flash.

Overshooting affects chemical mixing in red giants.

Abstract

We investigate the hydrodynamics of the core helium flash near its peak. Past research concerned with the dynamics of this event is inconclusive. However, the most recent multidimensional hydrodynamic studies suggest a quiescent behavior and seem to rule out an explosive scenario. Previous work indicated, that depending on initial conditions, employed turbulence models, grid resolution, and dimensionality of the simulation, the core helium flash leads either to the disruption of a low-mass star or to a quiescent quasi-hydrostatic evolution. We try to clarify this issue by simulating the evolution with advanced numerical methods and detailed microphysics. Assuming spherical or axial symmetry, we simulate the evolution of the helium core of a $1.25 M_{\odot}$ star with a metallicity Z=0.02 during the core helium flash at its peak with a grid-based hydrodynamics code. We find that the core helium flash neither rips the star apart, nor that it significantly alters its structure, as convection plays a crucial role in keeping the star in hydrostatic equilibrium. In addition, our simulations show the presence of overshooting, which implies new predictions concerning mixing of chemical species in red giants.