Electron capture and beta-decay rates for sd-shell nuclei in stellar environments relevant to high density O-Ne-Mg cores

Abstract

Electron capture and beta-decay rates for nuclear pairs in sd-shell are evaluated at high densities and high temperatures relevant to the final evolution of electron-degenerate O-Ne-Mg cores of stars with the initial masses of 8-10 solar mass. Electron capture induces a rapid contraction of the electron-degenerate O-Ne-Mg core. The outcome of rapid contraction depends on the evolutionary changes in the central density and temperature, which are determined by the competing processes of contraction, cooling, and heating. The fate of the stars are determined by these competitions, whether they end up with electron-capture supernovae or Fe core-collapse supernovae. Since the competing processes are induced by electron capture and beta-decay, the accurate weak rates are crucially important. The rates are obtained for pairs with A=20, 23, 24, 25 and 27 by shell-model calculations in sd-shell with the USDB Hamiltonian. Effects of Coulomb corrections on the rates are evaluated. The rates for pairs with A=23 and 25 are important for nuclear URCA processes that determine the cooling rate of O-Ne-Mg core, while those for pairs with A=20 and 24 are important for the core-contraction and heat generation rates in the core. We provide these nuclear rates at stellar environments in tables with fine enough meshes at various densities and temperatures for the studies of astrophysical processes sensitive to the rates. In particular, the accurate rate tables are crucially important for the final fates of not only O-Ne-Mg cores but also a wider range of stars such as C-O cores of lower mass stars.