The core helium flash revisited

Abstract

Context. We study turbulent convection during the core helium flash close to its peak by comparing the results of two and threedimensional 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 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 twodimensional 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.

Evolution of massive AGB stars

“…Our poor knowledge of convection presently limits our understanding of the structural and chemical evolution of SAGB stars. Only direct multidimensional simulations as courageously undertaken by Dearborn et al (2006), Herwig et al (2006), Meakin & Arnett (2007), Palacios & Brun (2008), Mocák et al (2009) and others will eventually clarify these critical aspects.…”

Asteroseismology and interferometry of the red giant star ϵ Ophiuchi

“…The critical factor in this starting model, apart from the total mass and the chemical composition at the tip of the giant branch, is the mass of the helium core. Hydrodynamical studies in 2D (Cole & Deupree 1983) and 3D (Mocàk et al 2009) confirm this picture, except for possible second order mixing effects due to turbulent overshoot at the convective-radiative interface, which cannot, at this point, be estimated precisely.…”

The chemical composition of carbon stars. The R-type stars

“…The situation is different for Z = 0 models Hollowell et al 1990) but in this case the mixing is directly linked to the lack of CNO elements in the H-shell and it cannot be extrapolated to more metalrich models 1 . Recently, two-and three-dimensional hydrodynamic simulations of the core He-flash in Population I stars have been performed Mocák et al 2008a,b;Mocák 2009). None of these simulations lead to substantial differences with respect to the hydrostatic case.…”