Influence of Rb73 on the ashes of accreting neutron stars

Abstract

We find that the proton separation energy, $S(p)$, of $^{73}\mathrm{Rb}$ is $\ensuremath{-}640(40)$ keV, deduced from the observation of $\ensuremath{\beta}$-delayed ground-state protons following the decay of $^{73}\mathrm{Sr}$. This lower-limit determination of the proton separation energy of $^{73}\mathrm{Rb}$ coupled with previous upper limits from nonobservation, provides a full constraint on the mass excess with $\mathrm{\ensuremath{\Delta}}M{(}^{73}\text{Rb})=\ensuremath{-}46.01\ifmmode\pm\else\textpm\fi{}0.04$ MeV. With this new mass excess and the excitation energy of the ${J}^{\ensuremath{\pi}}=5/{2}^{\ensuremath{-}}$ isobaric-analog state ($T=3/2$) in $^{73}\mathrm{Rb}$, an improved constraint can be put on the mass excess of $^{73}\mathrm{Sr}$ using the isobaric-multiplet mass equation (IMME), and we find $\mathrm{\ensuremath{\Delta}}M{(}^{73}\text{Sr})=\ensuremath{-}31.98\ifmmode\pm\else\textpm\fi{}0.37$ MeV. These new data were then used to study the composition of ashes on accreting neutron stars following Type I x-ray bursts. Counterintuitively, we find that there should be an enhanced fraction of $Ag102$ nuclei with more negative proton separation energies at the $^{72}\mathrm{Kr}$ rp-process waiting point. Larger impurities of heavier nuclei in the ashes of accreting neutron stars will impact the cooling models for such astrophysical scenarios.