A NEW URCA PROCESS

In the course of a general study of electron capture and beta-decay rates on fp shell nuclei which are abundant after core silicon burning in massive stars, we have found that the Fuller, Fowler, and Newman beta-decay rates are much stronger than generally has been realized. In fact, they can balance the capture rates during the conditions which are prevalent after core silicon burning, resulting in a new Urca process. The strength of the beta-decay rates is a result of thermal population of the Gamow-Teller back resonance in the parent nucleus and the behavior of beta-decay and electron capture Q-values for ensembles of nuclei in nuclear statistical equilibrium. All beta-decay rate tabulations prior to Fuller, Fowler, and Newman neglected the contribution of back resonances and thus drastically underestimated the overall rate. We use a simple analytic model to explain this balancing. The full rates are coupled to a nuclear statistical equilibrium code to demonstrate the strength of the decays. One zone models are used to examine, in a self-consistent fashion, how these rates could affect the presupernova evolution of the iron core and hence the core collapse problem.