HLRB Project h0071
Multidimensional simulations of the core helium flash in low mass stars
Max-Planck-Institut fuer Astrophysik
Proposing Institution
Max-Planck-Institut fuer Astrophysik
Project Manager
PD Dr. Ewald Müller
Karl-Schwarzschildstr. 1
85748 Garching
Abstract
In stars of less than about 2.5 solar masses the onset of heliumburning constitutes a major event, called the core helium flash. Thepre-flash stellar core has a white dwarf-like degenerate structurewith an off-center temperature maximum resulting from plasma- andphoto-neutrino cooling. When helium burning temperatures are reached,the liberated energy cannot be used to expand and cool the layers atthe temperature maximum but rather leads to a heating of the core anda strongly increasing nuclear energy release. Only when convectionsets in, part of the excess energy can be transported away by materialflow from the burning regions, inhibiting thereby a completethermonuclear explosion.While pre-flash evolution proceeds on a nuclear time scale of about100 million yrs, typical e-folding times for the energy release fromhelium burning become as low as hours at the peak of the flash, andhence are comparable to convective turnover times. Thus, the usualassumptions used in simple descriptions for convection in stellarevolution modeling (instantaneous mixing; time-independence) are nolonger valid. In addition, the assumption of hydrostatic equilibriumno longer needs to be fulfilled.Previous attempts to overcome these assumptions by allowing forhydrodynamic flow remained inconclusive. The results range from aconfirmation of the standard assumptions to a complete disruption ofthe star. Although the most recent (1996) and elaborate hydrodynamicstudy suggests that the flash does not produce a hydrodynamic event,there exists no coherent picture as to what extent and under whatcircumstances (stellar mass and composition) the core helium flashcould drastically differ from canonical stellar structurecalculations.We plan to reconsider the dynamics of the core helium flash, and toperform a comprehensive study of 2D and 3D hydrodynamic simulationsusing state-of-the-art initial models and numerical techniques, highresolution, detailed micro-physics, and a time-dependent gravitationalpotential. The evolution will be computed on a computational grid inspherical coordinates using an improved version of themulti-dimensional hydrodynamics code PROMETHEUS, which is based on adirect Eulerian implementation of the piecewise parabolic method, andwhich is OpenMP parallelized.In stars of less than about 2.5 solar masses the onset of helium burning constitutes a major event, called the core helium flash. The pre-flash stellar core has a white dwarf-like degenerate structure with an off-center temperature maximum resulting from plasma- and photo-neutrino cooling. When helium burning temperatures are reached, the liberated energy cannot be used to expand and cool the layers at the temperature maximum but rather leads to a heating of the core and a strongly increasing nuclear energy release. Only when convection sets in, part of the excess energy can be transported away by material flow from the burning regions, inhibiting thereby a complete thermonuclear explosion. While pre-flash evolution proceeds on a nuclear time scale of about 100 million yrs, typical e-folding times for the energy release from helium burning become as low as hours at the peak of the flash, and hence are comparable to convective turnover times. Thus, the usual assumptions used in simple descriptions for convection in stellar evolution modeling (instantaneous mixing; time-independence) are no longer valid. In addition, the assumption of hydrostatic equilibrium no longer needs to be fulfilled. Previous attempts to overcome these assumptions by allowing for hydrodynamic flow remained inconclusive. The results range from a confirmation of the standard assumptions to a complete disruption of the star. Although the most recent (2006) and elaborate hydrodynamic study suggests that the flash does not produce a hydrodynamic event, there exists no coherent picture as to what extent and under what circumstances (stellar mass and composition) the core helium flash could drastically differ from canonical stellar structure calcultions. We plan to reconsider the dynamics of the core helium flash, and to perform a comprehensive study of 2D and 3D hydrodynamic simulations using state-of-the-art initial models and numerical techniques, high resolution, detailed micro-physics, and a time-dependent gravitational potential. The evolution will be computed on a computational grid in a spherical cooridnates using the multi-dimensional hydrodynamics code HERAKLES, which is based on a direct Eulerian implementation of the piecewise parabolic method, and which is OpenMP parallelized.
