2025ApJ...988..255S

The unstable isotope ⁶⁰Fe, with a half-life of 2.6 million years, is produced primarily in supernova explosions. The observed presence of ⁶⁰Fe in cosmic rays and its detection in deep-sea crusts and sediments suggest two possible scenarios: either the direct acceleration of ⁶⁰Fe from supernova ejecta or its enrichment in the circumstellar material surrounding supernova progenitors, which indicates cosmic ray production in clusters of supernovae. Focusing on the latter scenario, we consider an environment shaped by successive supernova explosions, reminiscent of the Local Bubble around the time of the most recent supernova explosion. We independently tracked the evolution of the ⁶⁰Fe mass ratio within the Local Bubble using passive scalars. To investigate the spectra of protons and ⁶⁰Fe, we explicitly modeled cosmic-ray acceleration and transport at the remnant of the last supernova by simultaneously solving the hydrodynamical equations for the supernova outflow and the transport equations for cosmic rays, scattering turbulence, and large-scale magnetic field, using the time-dependent acceleration code Radiation Acceleration Transport Parallel Code. The main uncertainty in our prediction of the local ⁶⁰Fe flux at about pc = 1 GeV nuc^–1 is the magnetic-field structure in the Local Bubble and the cosmic-ray diffusion beyond the approximately 100 kyr of evolution covered by our study. We found that if the standard galactic propagation applies, the local ⁶⁰Fe flux would be around 3% of that measured. If there is a sustained reduction in the diffusion coefficient at and near the Local Bubble, then the expected ⁶⁰Fe flux could be up to 30% of that measured.