The Impact of the New ${}^{59}$Fe Decay Rates on ${}^{60}$Fe and ${}^{26}$Al Nucleosynthesis in Massive Stars

Abstract

The diffuse $\gamma$-ray emission from short-lived radioactive ${}^{26}$Al and ${}^{60}$Fe provides a direct probe of ongoing nucleosynthesis in the Galaxy. However, theoretical models have long struggled to reproduce the observed ${}^{60}$Fe/${}^{26}$Al flux ratio, typically predicting values significantly higher than constraints derived from INTEGRAL/SPI observations. In this work, we investigate the impact of the recently measured, temperature-dependent stellar ${\beta }^{-}$ decay rate of ${}^{59}$Fe on the nucleosynthesis of these isotopes. We compute a grid of non-rotating massive star models ($14$-$80$ M${}_{\odot }$) at solar metallicity using the MESA code, coupled with a rigorous numerical resolution analysis. We find that the updated rate significantly suppresses the net production of ${}^{60}$Fe by approximately 0.28 dex ($\sim 47\%$) compared to models using LMP theoretical rates, while leaving ${}^{26}$Al yields virtually unchanged. This reduction is primarily driven by the enhanced ${\beta }^{-}$ decay during convective carbon shell burning. Integrating these yields over a standard Salpeter Initial Mass Function, we predict a Galactic flux ratio of $\sim 0.18$, which is in excellent agreement with the observed value of $0.184\pm 0.042$. Furthermore, this ratio exhibits a weak dependence on the IMF slope. Our results indicate that the updated nuclear physics input significantly alleviates the long-standing ${}^{60}$Fe overproduction problem, bringing theoretical predictions into much closer alignment with current Galactic observations.

Accepted for publication in The Astrophysical Journal (ApJ)

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