Electron Capture in Stellar Collapse

Abstract

Massive stars in the range M ≥ 9 M_⊙, are thought to end in Type II supernova explosions. Once the elements up to iron have been reached in the thermonuclear burning, further fusion reactions are halted. The iron core, supported by degenerate electron pressure, develops an instability induced by a combination of nuclear photodisintegration and electron capture and implodes. The evolution of the capture rates on free protons and nuclei during the collapse phase is discussed. These change the two most important quantities in the collapse dynamics, namely the number of leptons per nucleon and the entropy. It is believed that high lepton fraction and low entropy are essential for shock wave formation and propagation, which eventually expels the outer parts of the star. Dynamical collapse calculations are discussed which indicate that the electron fraction per nucleon and the entropy at neutrino trapping densities are not changed significantly from their initial values.