The Final Evolution of ONeMg Electron-Degenerate Cores

Abstract

The final stages of the evolution of electron-degenerate ONeMg cores, resulting from carbon burning in `heavyweight" intermediate-mass stars (8 M_{0 } M 10 M<SUB>0</SUB>) and growing in mass either from carbon burning in a shell or from accretion of matter in a close binary system, are examined. When due account is taken of the Coulomb corrections, both in the equation of state and in the electron capture threshold energies, explosive NeO ignition takes place at densities high enough to ensure gravitational collapse to nuclear matter densities. It is shown that this result holds for two extreme assumptions concerning mixing in the presence of an overstable temperature gradient: no mixing (Ledoux criterion) and ordinary convective entropy mixing according to the Schwarzschild criterion (the latter delaying explosive ignition to still higher densities). Discrepancies among earlier calculations, due to omission of Coulomb corrections, are clarified with the use of the most recent electron capture rates on the relevant nuclides plus very finely zoned models.