On the effectiveness of Brink–Axel hypothesis for stellar weak rate calculations of heavy nuclei

Earlier weak rate calculations relied on the Brink–Axel hypothesis (BAH), but the assumption requires validation against microscopic calculations for reliable nucleosynthesis modeling. We investigate the effectiveness of BAH in computing stellar weak rates of ${\beta }^{-}$–decay and electron capture interactions in two sets of heavy neutron-rich nuclei: fp- and fpg-shell nuclei ($26\le Z\le 31$, $55\le A\le 82$) for post-silicon burning phases, and waiting-point nuclei ($24\le Z\le 81$, $74\le A\le 207$) for r-process. The impact of BAH is evaluated for allowed rates in the first set, and for unique first-forbidden and allowed rates in the second set. Our calculations used the proton–neutron quasi-particle random-phase approximation (pn-QRPA) model with two strength function prescriptions: one using state-by-state calculations and the other invoking the BAH, within stellar densities $(10^7\text{–}10^{11})$g/cm³ and temperatures $(5\text{–}30)$GK of post-silicon burning phase of massive stars. Our study reveals significant deviations between BAH-invoked and microscopic ${\beta }^{-}$–decay rates, with overestimation up to three orders of magnitude in allowed rates for fp/fpg-shell nuclei and for waiting-point nuclei exhibiting a stark difference in U1F rates up to four–five orders of magnitude, contrasting with relatively small deviations by up to an order of magnitude in electron capture rates. Our results suggest that application of BAH may lead to unrealistic weak stellar allowed (forbidden) ${\beta }^{-}$-decay rates.