Constraints for stellar electron-capture rates on ${}^{86}$Kr via the ${}^{86}$Kr($t$,${}^3$He$+{\hat{I}}^3$)${}^{86}$Br reaction and the implications for core-collapse supernovae

Abstract

In the late stages of stellar core-collapse, prior to core bounce, electron captures on medium-heavy nuclei drive deleptonization and simulations require the use of accurate reaction rates. Nuclei with neutron number near $N=50$, just above atomic number $Z=28$, play an important role, but rates used in astrophysical simulations rely primarily on a relatively simple single-state approximation. In order to improve the accuracy of astrophysical simulations, experimental data are needed to test the electron-capture rates and to guide the development of better theoretical models. This work presents the results of the ${}^{86}$Kr($t$,${}^3$He+${\hat{I}}^3$) experiment at the NSCL, from which an upper limit for the Gamow-Teller strength up to an excitation energy in ${}^{86}$Br of 5 MeV is extracted. The derived upper limit for the electron-capture rate on ${}^{86}$Kr indicates that the rate estimated through the single-state approximation is too high and that rates based on Gamow-Teller strengths estimated in shell-model and QRPA calculations are more accurate. The QRPA calculations tested in this manner were used for estimating the electron capture rates for 78 isotopes near $N=50$ and above $Z=28$. The impact of using these new electron-capture rates in simulations of supernovae instead of the rates based on the single-state approximation is investigated, indicating a significant reduction in the deleptonization that affects multi-messenger signals, such as the emission of neutrinos and gravitational waves.

15 pages, 10 figures