2011A&A...533A..53M

The injection of hydrogen into the convection shell powered by helium burning during the core helium flash is commonly encountered during the evolution of metal-free and extremely metal-poor low-mass stars. Multidimensional hydrodynamic simulations indicate that the hydrogen injection may also occur in more metal-rich stars due to turbulent entrainment that accelerates the growth of the shell convection zone and increases its size. However, one-dimensional stellar models cast doubts that helium-flash hydrogen mixing does occur as it requires the crossing of an entropy barrier at the helium-hydrogen interface. With specifically designed multidimensional hydrodynamic simulations, we aim to prove that an entropy barrier is no obstacle to the growth of the helium-burning shell convection zone in the helium core of a metal-rich Population I star, i.e.convection can penetrate into the hydrogen-rich layers for these stars, too. We study whether this is also possible in one-dimensional stellar evolutionary calculations. We artificially shift the hydrogen-rich layer closer to the outer edge of the helium-burning shell convection zone in a Population I star with a mass of 1.25M_☉, and simulate the subsequent evolution in two and three dimensions, respectively. We also perform stellar evolutionary calculations of the core helium flash in metal-rich stars implementing turbulent entrainment by means of a simple prescription. These simulations were performed with the Eulerian hydrodynamical code HERAKLES and the stellar evolution code STAREVOL, respectively. Our hydrodynamical simulations show that the helium-burning shell convection zone in the helium core moves across the entropy barrier and reaches the hydrogen-rich layers. This leads to a mixing of protons into the hotter layers of the core and to a rapid increase in the nuclear energy production at the upper edge of the helium-burning convection shell - the hydrogen injection flash. As a result, a second convection zone appears in the hydrogen-rich layers. In contrast to one-dimensional models, the entropy barrier separating the two convective shells from each other is largely permeable to chemical transport when allowing for multidimensional flow and consequently hydrogen is continuously mixed deep into the helium core. We find it difficult to replicate this behavior using one-dimensional stellar evolutionary calculations.