Astrophysical weak-interaction rates for selected A=20 and A=24 nuclei

We have evaluated the electron capture rates on Ne-20, F-20, Mg-24, and Na-24 and the beta decay rates for F-20 and Na-24 at temperature and density conditions relevant for the late-evolution stages of stars with M = 8M(circle dot) - 12M(circle dot) . The rates are based on recent experimental data and large-scale shell-model calculations. We show that the electron capture rates on Ne-20 and Mg-24 and the F-20 and Na-24 beta-decay rates are based on data in this astrophysical range, except for the capture rate on Ne-20, which we predict to have a dominating contribution from the second-forbidden transition between the Ne-20 and F-20 ground states in the density range log(10) rho Y-e (g cm(-3)) = 9.3-9.6. The dominance of a few individual transitions allows us to present the various rates by analytical expressions at the relevant astrophysical conditions. We also derive the screening corrections to the rates.