THE FINAL EVOLUTION OF ONEMG ELECTRON-DEGENERATE CORES

The final stages of the evolution of electron-degenerate ONeMg cores, resulting from carbon burning in ''heavyweight'' intermediate-mass sta rs (8 M(circle dot) less than or similar to M less than or similar to 10 M(circle dot)) and growing in mass either from carbon burning in a shell or from accretion of matter in a close binary system, are examin ed. When due account is taken of the Coulomb corrections, both in the equation of state and in the electron capture threshold energies, expl osive NeO ignition takes place at densities high enough to ensure grav itational collapse to nuclear matter densities. It is shown that this result holds for two extreme assumptions concerning mixing in the pres ence of an overstable temperature gradient: no mixing (Ledoux criterio n) and ordinary convective entropy mixing according to the Schwarzschi ld criterion (the latter delaying explosive ignition to still higher d ensities). Discrepancies among earlier calculations, due to omission o f Coulomb corrections, are clarified with the use of the most recent e lectron capture rates on the relevant nuclides plus very finely zoned models.