EDP Sciences
Free access
Volume 501, Number 2, July II 2009
Page(s) 659 - 677
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361/200811414
Published online 13 May 2009
A&A 501, 659-677 (2009)
DOI: 10.1051/0004-6361/200811414

The core helium flash revisited

II. Two and three-dimensional hydrodynamic simulations
M. Mocák1, 2, E. Müller1, A. Weiss1, and K. Kifonidis1

1  Max-Planck-Institut für Astrophysik, Postfach 1312, 85741 Garching, Germany
    e-mail: mmocak@mpa-garching.mpg.de
2  Institut d'Astronomie et d'Astrophysique, Université Libre de Bruxelles, CP 226, 1050 Brussels, Belgium
    e-mail: mmocak@ulb.ac.be

Received 25 November 2008 / Accepted 3 April 2009

Context. We study turbulent convection during the core helium flash close to its peak by comparing the results of two and three-dimensional hydrodynamic simulations.
Aims. In a previous study we found that the temporal evolution and the properties of the convection inferred from two-dimensional hydrodynamic studies are similar to those predicted by quasi-hydrostatic stellar evolutionary calculations. However, as vorticity is conserved in axisymmetric flows, two-dimensional simulations of convection are characterized by incorrect dominant spatial scales and exaggerated velocities. Here, we present three-dimensional simulations that eliminate the restrictions and flaws of two-dimensional models and that provide a geometrically unbiased insight into the hydrodynamics of the core helium flash. In particular, we study whether the assumptions and predictions of stellar evolutionary calculations based on the mixing-length theory can be confirmed by hydrodynamic simulations.
Methods. We used a multidimensional Eulerian hydrodynamics code based on state-of-the-art numerical techniques to simulate the evolution of the helium core of a 1.25 $M_{\odot}$ Pop I star.
Results. Our three-dimensional hydrodynamic simulations of the evolution of a star during the peak of the core helium flash do not show any explosive behavior. The convective flow patterns developing in the three-dimensional models are structurally different from those of the corresponding two-dimensional models, and the typical convective velocities are lower than those found in their two-dimensional counterparts. Three-dimensional models also tend to agree more closely with the predictions of mixing length theory. Our hydrodynamic simulations show the turbulent entrainment that leads to a growth of the convection zone on a dynamic time scale. In contrast to mixing length theory, the outer part of the convection zone is characterized by a subadiabatic temperature gradient.

Key words: stars: evolution -- hydrodynamics -- convection -- stars: interiors

© ESO 2009