Electron and positron capture rates on iron isotopes for core-collapse simulations

I present here the microscopic calculation of Gamow-Teller (GT) strength distributions, both in the electron capture and electron decay direction, for 54,55,56Fe. The associated electron and positron capture rates for these isotopes of iron are also calculated in stellar matter. In the core of massive stars isotopes of iron, 54,55,56Fe, are considered to be key players in decreasing the electron-to- baryon ratio (Ye) via electron capture and positron decay on these nuclide. On the other hand, positron capture and electron decay on 55Fe also has a consequential role in increasing the lepton ratio during the silicon burning phases of massive stars. The structure of the presupernova star is altered both by the changes in Ye and the entropy of the core material. The calculations are done using the proton-neutron quasiparticle random phase approximation (pn-QRPA) theory. The pn- QRPA theory allows a microscopic state-by-state calculation of stellar capture rates. Experimental deformation of nuclei are taken into account for the first time. Results are encouraging and are compared against measurements (where possible) and other calculations. The calculated electron capture rates are enhanced by around an order of magnitude for 55Fe during the oxygen and silicon shell burning stages of massive stars as compared to the large-scale shell model calculations. For the case of even-even isotopes 54,56Fe the calculated electron capture rates are in very good agreement with the shell model results. (author)