Electron capture in stellar collapse

Abstract

Electron-capture rates are calculated for the high-density regime of pre-supernova stellar collapse. The Gamow-Teller contribution arises from thermal unblocking and proceeds mainly through captures beneath the T = 0 energy threshold. Parity-forbidden (Δl = 1) processes also contribute, particularly the 2⁻ unique forbidden transition. Both are quenched by the effective renormalization of the vector and axial vector charges by ground-state correlations and isobars. Special attention is paid to the temperature-induced unblocking of the GT's and the thermal redistribution of the forbidden-transition strength. A statistical treatment averages the contributions of nuclear species present in the mass range 60 < A < 100. Dynamical collapse calculations indicate that the electron fraction at neutrino-trapping densities is not much reduced from its initial value. In contrast to previous work, the entropy is shown to decrease due to the below threshold GT captures as well as those on free protons. The small amount of electron capture and the entropy production are both useful in producing successful supernova explosions.