Short-lived radionuclides (SLRs) recorded in meteorites probe the nucleosynthesis events that preceded the Sun's birth. Galactic chemical evolution (GCE) models by Trueman et al. (2025) show that the early Solar System (ESS) abundances of the explosive SLRs 60^{60}Fe and 53^{53}Mn can, within current uncertainties, be inherited from the Galactic background together with the s-process SLRs 107^{107}Pd and 182^{182}Hf — but the models predict an interstellar 60^{60}Fe/53^{53}Mn ratio at the time of Solar System formation that is completely inconsistent with the ESS value. The single identified lever, a larger fraction of faint high-fallback core-collapse supernovae (CCSNe) than usually assumed, is unconstrained, as are additional 60^{60}Fe sources (electron-capture supernovae and super-AGB stars), the 53^{53}Mn yield of Type Ia supernovae, and the recurrence times needed for stochastic enrichment modeling. The open problem is to decide whether a purely GCE origin can explain the ESS abundances of the modeled SLR suite (60^{60}Fe, 53^{53}Mn, 107^{107}Pd, and 182^{182}Hf), or whether the inconsistency rules it out; the missing sources listed above are the unconstrained inputs that Trueman et al. (2025) identify as bearing on that decision. A 2026 follow-up by the same group treats steady-state scenarios but leaves every one of these inputs unresolved.

Background

Short-lived radionuclides (SLRs) are radioactive isotopes with half-lives of roughly 10510^5–10810^8 years whose decay products are found trapped in meteoritic minerals, proving they were alive in the early Solar System (ESS) when the first solids formed. The first solids, calcium-aluminium-rich inclusions (CAIs), define time zero for the Solar System; the abundances of SLRs in later-forming components, time-corrected back to CAI formation, give the ESS reference ratios (e.g. 60^{60}Fe/56^{56}Fe ≈7.7×10−9\approx 7.7\times10^{-9} from the recent Fang et al. (2025) determination, superseding the earlier ≈1.0×10−8\approx 1.0\times10^{-8} value adopted in the Trueman et al. (2025) model comparison; 53^{53}Mn/55^{55}Mn ≈7×10−6\approx 7\times10^{-6}; the corresponding ESS radio-to-radio ratio quoted in that comparison is 60^{60}Fe/53^{53}Mn =0.127−0.038+0.045=0.127^{+0.045}_{-0.038} at 2σ2\sigma). Because SLR half-lives are comparable to the time a Sun-like star takes to form inside its birth cloud, these isotopes act as cosmochronometers: comparing the ESS abundances to the theoretical interstellar-medium (ISM) abundances yields the 'isolation time' tisot_{\rm iso} during which the presolar molecular cloud was cut off from fresh stellar ejecta before the Sun formed. 53^{53}Mn (half-life 3.74 Myr) is produced by explosive oxygen and partial silicon burning in both core-collapse supernovae (CCSNe, the gravitational collapse of massive stars) and Type Ia supernovae (SNe Ia, thermonuclear explosions of white dwarfs); 60^{60}Fe (half-life 2.62 Myr) is produced exclusively by CCSNe, mainly by neutron captures in the carbon- and helium-burning layers of the progenitor. Two rarer proposed 60^{60}Fe sources, usually omitted from GCE calculations because their rates are poorly known, are electron-capture supernovae (ECSNe; collapses of stars with oxygen–neon–magnesium cores triggered by electron capture on magnesium) and super-AGB stars (super-asymptotic-giant-branch stars, the most massive stars, with initial masses of roughly 8–10 solar masses, to undergo asymptotic-giant-branch evolution, ending either as ECSNe or as oxygen–neon white dwarfs). Galactic chemical evolution (GCE) models follow how such ejecta, with mass- and metallicity-dependent yields, build up SLR-to-stable abundance ratios in the ISM over the history of the Milky Way. The key free inputs are the supernova yield sets; the mix of SN Ia progenitor channels (sub-Chandrasekhar-mass vs near-Chandrasekhar-mass white dwarfs) and their delay-time distribution (DTD, the wait between stellar formation and explosion); the gas-inflow history; and the parameter KK describing the star-formation efficiency assumed. Because enrichment events are discrete, the ISM abundance of an SLR fluctuates stochastically; the spread depends on the ratio of the SLR mean life τ\tau to the average recurrence time γ\gamma between enrichment events, and γ\gamma per source class is itself poorly constrained. Two observational anchors sharpen the problem: gamma-ray lines from decaying 60^{60}Fe and 26^{26}Al, detected across the Galaxy by the INTEGRAL/SPI gamma-ray spectrometer, fix the observed 60^{60}Fe/26^{26}Al flux ratio, which GCE models already tend to overpredict; and deep-sea sediments record recent 60^{60}Fe and 53^{53}Mn influxes from nearby supernovae. In this framework, Trueman et al. (2022) had shown that the SLRs 107^{107}Pd and 182^{182}Hf, produced by the s-process (slow neutron capture in evolved stars), admit a self-consistent GCE origin. Trueman et al. (2025) extended the analysis to 60^{60}Fe and 53^{53}Mn with a grid of GCE models varying CCSN yield sets, SN Ia channels, DTDs, and KK, together with an exploratory case in which the SN Ia 53^{53}Mn yield is set to zero so that 53^{53}Mn in the ESS would originate from a single source class. They found overlapping isolation-time distributions (most likely tiso∼9t_{\rm iso}\sim9–12 Myr) across 53^{53}Mn, 60^{60}Fe, 107^{107}Pd, and 182^{182}Hf within current uncertainties — so no local stellar source is required — but with a systematic offset: for the ESS 60^{60}Fe/56^{56}Fe value adopted in that comparison, the maximum likelihood for 60^{60}Fe sits ∼4\sim4–5 Myr below that for 53^{53}Mn (using the newer Fang et al. (2025) value instead shifts the 60^{60}Fe isolation-time distribution by about +1+1 Myr, alleviating but not removing the gap), and, decisively, the predicted ISM 60^{60}Fe/53^{53}Mn ratio at Solar formation is completely inconsistent with the ESS value (about 90% of models underpredict it for that adopted ESS ratio, implying formally negative isolation times, while the remainder give isolation times below 1 Myr). The mismatch points to the relative yields of the inner explosive-burning ejecta (source of 53^{53}Mn, plausibly removed by fallback of inner material onto the compact remnant in faint CCSNe) versus the outer neutron-capture layers (source of 60^{60}Fe).

Problem Statement

Determine whether a purely Galactic-chemical-evolution origin — inheritance of SLRs from the Galactic background ISM, with no late local injection — can account for the ESS abundances of the explosive SLRs 60^{60}Fe and 53^{53}Mn jointly with the previously modeled s-process SLRs 107^{107}Pd and 182^{182}Hf, given that current GCE models predict an ISM 60^{60}Fe/53^{53}Mn ratio at the time of Solar System formation that is inconsistent with the ESS-inferred value. Trueman et al. (2025) identify the following missing-source physics as bearing on this overarching question, each as an input to the single determination stated above rather than as a separate question: (1) the occurrence frequency and characteristic 60^{60}Fe and 53^{53}Mn yields of faint, high-fallback CCSNe (core-collapse supernovae whose inner, 53^{53}Mn-rich ejecta fall back onto the compact remnant instead of being expelled), and whether a physically plausible larger fraction of them reconciles the predicted 60^{60}Fe/53^{53}Mn ratio and the offset between the 60^{60}Fe and 53^{53}Mn isolation-time distributions (∼4\sim4–5 Myr for the ESS 60^{60}Fe/56^{56}Fe value adopted in that comparison, about 1 Myr smaller with the newer Fang et al. (2025) value) with the ESS values; (2) the 60^{60}Fe yields and Galactic rates of electron-capture supernovae (collapses of oxygen–neon–magnesium cores triggered by electron capture) and super-AGB stars (the most massive asymptotic-giant-branch stars, of initial mass roughly 8–10 solar masses), including whether adding them improves the multi-SLR solution without violating the observed 60^{60}Fe/26^{26}Al gamma-ray flux ratio; (3) whether SNe Ia contribute 53^{53}Mn to the ESS, i.e. realistic 53^{53}Mn yields for the actual progenitor-channel mix (sub-Chandrasekhar vs near-Chandrasekhar) and delay-time distributions, testing the exploratory zero-SN-Ia assumption; and (4) the enrichment recurrence times γ\gamma per source class (the average wait between enrichment events) and whether they can be constrained sufficiently to model stochastic abundance fluctuations, so that τ/γ\tau/\gamma — the ratio of the SLR mean life τ\tau to γ\gamma, which controls the size of the fluctuations — and the ESS-consistency likelihoods can be computed for multi-source SLRs; Trueman et al. (2025) instead adopt, for each SLR, the largest γ\gamma for which τ/γ≥2\tau/\gamma\ge2, which yields the most conservative isolation-time errors.

References

  1. Trueman, T. C. L.; Yagüe López, A.; Lugaro, M.; Pignatari, M. (2025). Galactic Chemical Evolution of Short-lived Radioactive Isotopes Produced by Explosive Nucleosynthesis: 60Fe and 53Mn. The Astronomical Journal 171, 16. DOI: 10.3847/1538-3881/ae0c99. URL: https://doi.org/10.3847/1538-3881/ae0c99 (preprint arXiv:2509.20956, DOI: 10.48550/arXiv.2509.20956)
  2. Soós, B.; Trueman, T. C. L.; Yagüe López, A.; Roberti, L.; Lugaro, M. (2026). An Updated Picture of Presolar History from Short-lived Radioactive Isotopes and Inferences on the Birth of the Sun. The Astrophysical Journal 1001, 58. DOI: 10.3847/1538-4357/ae4c8b. URL: https://doi.org/10.3847/1538-4357/ae4c8b (preprint arXiv:2604.00719, URL: https://arxiv.org/abs/2604.00719)
  3. Fang, L.; Moynier, F.; Chaussidon, M.; Limare, A.; Makhatadze, G. V.; Villeneuve, J. (2025). The initial solar system abundance of 60Fe and early core formation of the first asteroids. Science Advances 11, eadp9381. DOI: 10.1126/sciadv.adp9381. URL: https://doi.org/10.1126/sciadv.adp9381
  4. Trueman, T. C. L.; Côté, B.; Yagüe López, A.; den Hartogh, J.; Pignatari, M.; Soós, B.; Karakas, A. I.; Lugaro, M. (2022). Galactic Chemical Evolution of Radioactive Isotopes with an s-process Contribution. The Astrophysical Journal 924, 10. DOI: 10.3847/1538-4357/ac31b0. URL: https://doi.org/10.3847/1538-4357/ac31b0
  5. Tang, H.; Dauphas, N. (2015). Low 60Fe abundance in Semarkona and Sahara 99555. The Astrophysical Journal 802, 22. DOI: 10.1088/0004-637X/802/1/22. URL: https://doi.org/10.1088/0004-637X/802/1/22
  6. Tissot, F. L. H.; Dauphas, N.; Grove, T. L. (2017). Distinct 238U/235U ratios and REE patterns in plutonic and volcanic angrites: Geochronologic implications and evidence for U isotope fractionation during magmatic processes. Geochimica et Cosmochimica Acta 213, 593–617. DOI: 10.1016/j.gca.2017.06.045. URL: https://doi.org/10.1016/j.gca.2017.06.045
  7. Wang, W.; Siegert, T.; Dai, Z. G.; Diehl, R.; Greiner, J.; Heger, A.; Krause, M.; Lang, M.; Pleintinger, M. M. M.; Zhang, X. L. (2020). Gamma-Ray Emission of 60Fe and 26Al Radioactivity in Our Galaxy. The Astrophysical Journal 889, 169. DOI: 10.3847/1538-4357/ab6336. URL: https://doi.org/10.3847/1538-4357/ab6336
  8. Wanajo, S.; Janka, H.-T.; Müller, B. (2013). Electron-capture Supernovae as Sources of 60Fe. The Astrophysical Journal 774, L6. DOI: 10.1088/2041-8205/774/1/L6. URL: https://doi.org/10.1088/2041-8205/774/1/L6
  9. Jones, S. W.; Möller, H.; Fryer, C. L.; Fontes, C. J.; Trappitsch, R.; Even, W. P.; Couture, A.; Mumpower, M. R.; Safi-Harb, S. (2019). 60Fe in core-collapse supernovae and prospects for X-ray and gamma-ray detection in supernova remnants. Monthly Notices of the Royal Astronomical Society 485, 4287–4310. DOI: 10.1093/mnras/stz536. URL: https://doi.org/10.1093/mnras/stz536
  10. Lugaro, M.; Doherty, C. L.; Karakas, A. I.; Maddison, S. T.; Liffman, K.; García-Hernández, D. A.; Siess, L.; Lattanzio, J. C. (2012). Short-lived radioactivity in the early solar system: The Super-AGB star hypothesis. Meteoritics & Planetary Science 47, 1998–2012. DOI: 10.1111/j.1945-5100.2012.01411.x. URL: https://doi.org/10.1111/j.1945-5100.2012.01411.x
  11. Seitenzahl, I. R.; Ciaraldi-Schoolmann, F.; Röpke, F. K.; Fink, M.; Hillebrandt, W.; Kromer, M.; Pakmor, R.; Ruiter, A. J.; Sim, S. A.; Taubenberger, S. (2013). Three-dimensional delayed-detonation models with nucleosynthesis for Type Ia supernovae. Monthly Notices of the Royal Astronomical Society 429, 1156–1172. DOI: 10.1093/mnras/sts402. URL: https://doi.org/10.1093/mnras/sts402
  12. Côté, B.; Yagüe, A.; Világos, B.; Lugaro, M. (2019). Stochastic Chemical Evolution of Radioactive Isotopes with a Monte Carlo Approach. The Astrophysical Journal 887, 213. DOI: 10.3847/1538-4357/ab5a88. URL: https://doi.org/10.3847/1538-4357/ab5a88
  13. Wehmeyer, B.; Yagüe López, A.; Côté, B.; Pető, M. K.; Kobayashi, C.; Lugaro, M. (2023). Inhomogeneous Enrichment of Radioactive Nuclei in the Galaxy: Deposition of Live 53Mn, 60Fe, 182Hf, and 244Pu into Deep-sea Archives. Surfing the Wave? The Astrophysical Journal 944, 121. DOI: 10.3847/1538-4357/acafec. URL: https://doi.org/10.3847/1538-4357/acafec
  14. Wehmeyer, B.; Yagüe López, A.; Côté, B.; Pető, M. K.; Kobayashi, C.; Lugaro, M. (2025). Galactic chemical evolution with the short-lived isotopes Mn-53, Fe-60, Hf-182, and Pu-244. arXiv:2510.13070. DOI: 10.48550/arXiv.2510.13070. URL: https://arxiv.org/abs/2510.13070
  15. Kaur, T.; Sahijpal, S. (2019). Heterogeneous evolution of the galaxy and the origin of the short-lived nuclides in the early solar system. Monthly Notices of the Royal Astronomical Society 490, 1620–1640. DOI: 10.1093/mnras/stz2720. URL: https://doi.org/10.1093/mnras/stz2720
  16. Korschinek, G.; Faestermann, T.; Poutivtsev, M.; Arazi, A.; Knie, K.; Rugel, G.; Wallner, A. (2020). Supernova-Produced 53Mn on Earth. Physical Review Letters 125, 031101. DOI: 10.1103/PhysRevLett.125.031101. URL: https://doi.org/10.1103/PhysRevLett.125.031101
  17. Spyrou, A.; Richman, D.; Couture, A.; Fields, C. E.; Liddick, S. N.; Childers, K.; Crider, B. P.; DeYoung, P. A.; Dombos, A. C.; Gastis, P.; Guttormsen, M.; Hermansen, K.; Larsen, A. C.; Lewis, R.; Lyons, S.; Midtbø, J. E.; Mosby, S.; Muecher, D.; Naqvi, F.; Palmisano-Kyle, A.; Perdikakis, G.; Prokop, C.; Schatz, H.; Smith, M. K.; Sumithrarachchi, C.; Sweet, A. (2024). Enhanced production of 60Fe in massive stars. Nature Communications 15, 9608. DOI: 10.1038/s41467-024-54040-4. URL: https://doi.org/10.1038/s41467-024-54040-4
  18. Falla, A.; Roberti, L.; Limongi, M.; Chieffi, A. (2025). The role of rotation on the yields of the two gamma-ray emitters 26Al and 60Fe ejected by massive stars. arXiv:2508.07705. DOI: 10.48550/arXiv.2508.07705. URL: https://arxiv.org/abs/2508.07705
  19. Battino, U.; Lederer-Woods, C.; Pignatari, M.; Soós, B.; Lugaro, M.; Vescovi, D.; Cristallo, S.; Woods, P. J.; Karakas, A. (2026). Impact of newly measured 26Al(n,p)26Mg and 26Al(n,alpha)23Na reaction rates on the nucleosynthesis of 26Al in stars. arXiv:2601.05710. DOI: 10.48550/arXiv.2601.05710. URL: https://arxiv.org/abs/2601.05710
  20. Miller, N.; Joyce, M.; Johnson, C. I.; Tayar, J.; Trueman, T.; Rich, R. M. (2026). The Two-infall Model Revisited: Constraints on Milky Way Bulge Assembly from >30,000 Galactic Chemical Evolution Models and Machine Learning. The Astrophysical Journal 1006, 41. DOI: 10.3847/1538-4357/ae76f7. URL: https://doi.org/10.3847/1538-4357/ae76f7