Electron-capture Rates in 20Ne for a Forbidden Transition to the Ground State of 20F Relevant to the Final Evolution of High-density O-Ne-Mg Cores

Electron capture on Ne-20 is critically important for the final stage of evolution of stars with initial masses of 8-10 M-circle dot. In the present paper, we evaluate electron capture rates for a forbidden transition Ne-20 (0(g.s.)(+)) -> F-20 (2(g.s.)(+))in stellar environments by the multipole expansion method with the use of shell-model Hamiltonians. These rates have not been accurately determined in theory as well as in experiments. Our newly evaluated rates are compared with those obtained by a prescription that treats the transition as an allowed Gamow-Teller transition with the strength determined from a recent beta-decay experiment for F-20 (2(g.s.)(+)) -> Ne-20 (0(g.s.)(+)). We find that different electron energy dependence of the transition strengths between the two methods leads to sizable differences in the weak rates of the two methods. We also find that the Coulomb effects, that is, the effects of screening on ions and electrons are nonnegligible. We apply our electron-capture rates on Ne-20 to the calculation of the evolution of high-density O-Ne-Mg cores of 8-10 M-circle dot stars. We find that our new rates affect the abundance distribution and the central density at the final stage of evolution.