Evolution of 8-10 solar mass stars toward electron capture supernovae. II - Collapse of an O + NE + MG core

Abstract

In this paper, the helium core of an 8.8 solar mass star is evolved from the helium-burning stage through the early stage of collapse of an O + Ne + Mg core. The star undergoes helium and carbon burning under nondegenerate conditions and leaves an O + Ne + Mg core. The penetration of the surface convection zone into the helium layer starts much earlier than in a 9.6 solar mass star studied elsewhere. The subsequent evolution is brought about by hydrogen-helium double-shell burning. The mass interior to the helium-burning shell, M(Heb), increases toward the Chandrasekhar limit. After M(Heb) reaches 1.375 solar mass, electron captures on Mg-24, Na-24, Ne-20, and F-20 take place. The electron concentration decreases, the resulting rapid core contraction ignites the oxygen deflagration, and the material undergoes incineration into nuclear statistical equilibrium at the oxygen deflagration front. The collapse of the core is accelerated, and the oxygen deflagration front advances.