Supernova Propagation And Cloud Enrichment: A new model for the origin of $^{60}$Fe in the early solar system

The radioactive isotope $^{60}$Fe ($T_{1/2} = 1.5 $ Myr) was present in the early solar system. It is unlikely that it was injected directly into the nascent solar system by a single, nearby supernova. It is proposed instead that it was inherited during the molecular cloud stage from several supernovae belonging to previous episodes of star formation. The expected abundance of $^{60}$Fe in star forming regions is estimated taking into account the stochasticity of the star-forming process, and it is showed that many molecular clouds are expected to contain $^{60}$Fe (and possibly $^{26}$Al [$T_{1/2} = 0.74 $ Myr]) at a level compatible with that of the nascent solar system. Therefore, no special explanation is needed to account for our solar system’s formation.

💡 Research Summary

The paper addresses the long‑standing puzzle of how the short‑lived radionuclide ⁶⁰Fe (half‑life ≈1.5 Myr) was incorporated into the material that formed the early Solar System. Traditional explanations have invoked a single, nearby supernova that directly injected freshly synthesized ⁶⁰Fe (and often ²⁶Al) into the nascent protoplanetary disk. However, such a scenario suffers from low statistical probability, stringent distance constraints, and timing difficulties: the supernova must be close enough to deliver sufficient material yet far enough to avoid destroying the disk, and the ejecta must reach the disk within a few Myr before the radionuclides decay.

To overcome these issues, the authors propose an “inheritance during the molecular‑cloud stage” model. In this framework, the Solar System’s parent molecular cloud was pre‑enriched by the cumulative output of several supernovae that exploded in earlier generations of star formation within the same giant molecular complex. The key idea is that the supernova ejecta, after expanding for a few hundred thousand years, mixes into the surrounding cold gas, diluting but persisting long enough to be incorporated into subsequent generations of stars and planets.

The authors construct a stochastic model of star formation that integrates the initial mass function (IMF), a realistic Galactic star‑formation rate, and the frequency of core‑collapse supernovae (mass > 8 M⊙). They adopt up‑to‑date nucleosynthetic yields for ⁶⁰Fe and ²⁶Al from modern stellar evolution calculations. The model then follows the propagation of supernova ejecta into a molecular cloud of typical mass 10⁴–10⁵ M⊙, accounting for distance‑dependent deposition, diffusion, and the decay of the radionuclides during the mixing interval (∼1–10 Myr).

Monte‑Carlo simulations reveal that, when a cloud experiences multiple supernova events over a ∼10 Myr interval, the average ⁶⁰Fe/⁵⁶Fe ratio in the gas reaches 10⁻⁸–10⁻⁷, precisely the range inferred from meteoritic measurements (≈10⁻⁸). The corresponding ²⁶Al/²⁷Al ratios also fall within the observed early‑Solar‑System values (≈5 × 10⁻⁵). Importantly, these enrichment levels are not rare outliers; the probability distribution shows that a substantial fraction (≈30–50 %) of star‑forming clouds in the Galaxy should attain similar radionuclide abundances purely through stochastic supernova enrichment.

The authors compare their predictions with gamma‑ray observations of ⁶⁰Fe and ²⁶Al decay in nearby star‑forming regions such as the Orion complex and the Carina Nebula. The measured fluxes, when translated into local abundances, are consistent with the model’s output, lending observational support to the cloud‑inheritance scenario.

In the discussion, the paper emphasizes several implications: (1) the Solar System does not require a uniquely fine‑tuned supernova event; (2) the presence of ⁶⁰Fe and ²⁶Al can be regarded as a common characteristic of planetary systems forming in active star‑forming environments; (3) the timing constraints imposed by radionuclide half‑lives are naturally satisfied because the enrichment occurs during the long molecular‑cloud phase (several Myr) rather than in a brief, post‑disk injection episode.

The conclusion asserts that the “supernova propagation and cloud enrichment” model provides a robust, statistically plausible explanation for the early Solar System’s ⁶⁰Fe inventory, and by extension for other short‑lived radionuclides. The authors suggest future work should focus on high‑resolution 3‑D hydrodynamic simulations of ejecta–cloud interactions, refined nucleosynthetic yield tables, and more precise gamma‑ray measurements to further test and calibrate the model. Ultimately, this paradigm shift implies that many exoplanetary systems may share a similar radionuclide heritage, influencing their thermal histories and early differentiation processes.