Enhanced production of ⁶⁰Fe in massive stars

Massive stars are a major source of chemical elements in the cosmos, ejecting freshly produced nuclei through winds and core-collapse supernova explosions into the interstellar medium. Among the material ejected, long-lived radioisotopes, such as ⁶⁰Fe (iron) and ²⁶Al (aluminum), offer unique signs of active nucleosynthesis in our galaxy. There is a long-standing discrepancy between the observed ⁶⁰Fe/²⁶Al ratio by γ-ray telescopes and predictions from supernova models. This discrepancy has been attributed to uncertainties in the nuclear reaction networks producing ⁶⁰Fe, and one reaction in particular, the neutron-capture on ⁵⁹Fe. Here we present experimental results that provide a strong constraint on this reaction. We use these results to show that the production of ⁶⁰Fe in massive stars is higher than previously thought, further increasing the discrepancy between observed and predicted ⁶⁰Fe/²⁶Al ratios. The persisting discrepancy can therefore not be attributed to nuclear uncertainties, and points to issues in massive-star models.