Evolution of 8-10 solar mass stars toward electron capture supernovae. I - Formation of electron-degenerate O + NE + MG cores

Abstract

One triggering mechanism suggested for supernovae involves the collapse of a degenerate core due to electron captures. Barkat et al. (1974) showed that an eight solar mass star develops a degenerate core after nonexplosive carbon burning. Miyaji et al. (1980) performed hydrodynamical calculations regarding a degenerate O + Ne + Mg core. It was found that electron captures on Mg-24 and Ne-20 trigger the collapse of the core prior to the initiation of explosive oxygen burning. In the present investigation the gravitational contraction of hypothetical pure neon stars is discussed in order to clarify the effect of electron degeneracy on neon ignition and stellar evolution, which is crucial for 8-10 solar mass stars. A summary is provided of the evolution from helium burning through the phase of the developing O + Ne + Mg core. Attention is given to neon ignition, the origin of hydrogen-deficient carbon stars, and the evolution toward supernova stages, and possible formation of white dwarfs.