Investigation of important weak interaction nuclei in presupernova evolution

Abstract

The project aims to investigate most important weak interaction nuclei in the presupernova evolution of massive stars. To achieve the goal, an ensemble containing 728 nuclei in the mass range A = 1--100 was considered. We computed the mass fractions of these nuclei using Saha's equation for predetermined values of \textit{$T$}, \textit{$ρ$} and \textit{$Y_e$} and assuming nuclear statistical equilibrium. The nuclear partition functions were obtained using a newly introduced recipe where excited states, up to 10 $MeV$, were treated as discrete. The weak interaction rates (electron capture (\textit{ec}) and $β$-decay (\textit{bd})) were calculated in a \textit{totally} microscopic fashion using the proton-neutron quasiparticle random phase approximation (pn-QRPA) model and without assuming the Brink-Axel hypothesis. The calculated rates were coupled with the computed mass fractions to investigate the time rate of change of lepton to baryon fraction of the stellar matter. We compare our results with the previous calculations reported in the literature. Noticeable differences up to orders of magnitude are reported with previous calculations. These differences may influence the evolution of the star in the later stages of presupernova. We present a list of top 50 \textit{ec} and \textit{bd} nuclei which have the largest effect on $Y_e$ for conditions after silicon core burning. The competition between the \textit{ec} and \textit{bd} rates in stellar core was investigated and it was found that $Y_e$ = 0.424--0.455 is the interval where the \textit{bd} results are bigger than the \textit{ec} rates.