Short-lived radionuclides (SLRs) in early solar system solids suggest fresh nucleosynthetic input near the Sun's birth. Banerjee, Qian, Heger and Haxton (Nature Communications 7, 13639, 2016) proposed that an ~11.8 solar-mass core-collapse supernova (CCSN) both triggered the collapse of the protosolar cloud and supplied several SLRs, notably explaining the presence of 10^{10}Be (half-life 1.4 Myr) through neutrino-induced nucleosynthesis in the carbon shell. They identified a linked program to test the hypothesis: resolving the discrepant early-solar-system 60^{60}Fe/56^{56}Fe determinations, pinning down the thermally enhanced stellar beta-decay rate of 181^{181}Hf that controls the 182^{182}Hf yield, replacing one-dimensional parametrized models with self-consistent multi-dimensional simulations quantifying explosion energetics and fallback, modeling the astrophysical environment (26^{26}Al enrichment by winds from massive stars in an evolving giant molecular cloud, cosmic-ray production in the CCSN remnant, late solar energetic particle irradiation), and measuring Li, Be and B isotopes in meteorites as discriminants. Later literature has resolved part of this program — the 60^{60}Fe/56^{56}Fe discrepancy settled toward ≈10−8≈10^{-8}, and 3D simulations of the same 11.8-solar-mass progenitor find no fallback while yielding a 10^{10}Be production too low to match meteoritic values — but the central question remains open: can the low-mass CCSN trigger scenario be made self-consistent, or is it excluded by the updated constraints?

Background

When the solar system formed 4.57 Gyr ago, its first solids (calcium-aluminum-rich inclusions, CAIs, in primitive meteorites) incorporated short-lived radionuclides (SLRs) — radioactive nuclei with half-lives of ≈0.1≈0.1–100 Myr, such as 10^{10}Be, 26^{26}Al, 36^{36}Cl, 41^{41}Ca, 53^{53}Mn, 60^{60}Fe, 107^{107}Pd, 135^{135}Cs, 182^{182}Hf and 205^{205}Pb — whose decay products are recorded as isotope anomalies. Because SLRs decay on timescales short compared with Galactic mixing times, they require nucleosynthesis shortly before or during solar system formation, and their abundances are a forensic probe of the Sun's birth environment. Neutron-capture nucleosynthesis comes in a slow (s) process, in low-neutron-density sites such as asymptotic giant branch (AGB) stars and massive-star helium shells, and a rapid (r) process, in neutron-rich explosive sites; the attribution of 182^{182}Hf between these determines how it is used as an early chronometer. A core-collapse supernova (CCSN) also produces light nuclei such as 7^{7}Li, 11^{11}B and 10^{10}Be through the neutrino process (ν-process), in which supernova neutrinos inelastically scatter on nuclei in the outer shells. The classic proposal of Cameron and Truran (1977) is that a nearby supernova shock both triggered the collapse of the protosolar cloud and injected freshly made SLRs into it; a competing class of explanations attributes the SLRs to irradiation of nebular gas by solar energetic particles (SEPs, MeV-to-GeV ions emitted by the young Sun), to inheritance from the ambient molecular cloud, or to the ordinary chemical evolution of the Galaxy. Banerjee, Qian, Heger and Haxton (2016) sharpened the supernova-trigger idea: for a low-mass progenitor of ~11.8 solar masses (zero-age main sequence), the ν-process 10^{10}Be yield from 12^{12}C is comparatively insensitive to progenitor mass, and a dilution factor of f≈5×10−4f≈5×10^{-4} (the fraction of the supernova's radionuclide yield injected into the protosolar cloud) and a decay interval of Δ≈1Δ≈1 Myr (the time between nucleosynthesis in the ejecta and incorporation into the first solar-system solids) can simultaneously account for the measured 10^{10}Be, 41^{41}Ca and 107^{107}Pd without exceeding other SLR abundances, while leaving stable-isotope shifts of Mg, Si, Ca and Fe below the meteoritic limit of ≈1≈1%. Two internal knobs were required: modest fallback (late re-accretion of the innermost shocked zones onto the newly formed neutron star, which removes the overproduced 53^{53}Mn and 60^{60}Fe), and external enrichment of 26^{26}Al in the swept-up interstellar medium by winds of nearby massive stars that exploded weakly or not at all. The 2016 calculation was one-dimensional with a parametrized explosion energy and piston-driven ejecta; its authors explicitly identified what further work was needed to confirm or falsify the scenario. The problem is judged against the early-solar-system (ESS) abundance scale in which, following decisive measurements and a community reassessment (Tang and Dauphas 2015; Trappitsch et al. 2018; Diehl et al. 2021), the recommended initial 60^{60}Fe/56^{56}Fe is (1.01±0.27)×10−8(1.01±0.27)×10^{-8} rather than the ≈5×10−7≈5×10^{-7} values from secondary-ion mass spectrometry of chondrules that were still in play in 2016.

Problem Statement

Determine whether the formation of the solar system was triggered by a low-mass core-collapse supernova (CCSN; zero-age main-sequence mass of roughly 11-12 solar masses, exemplified by the 11.8 solar-mass model of Banerjee et al. 2016) whose ejecta were injected into the protosolar cloud — or establish that this scenario is excluded by the evidence. Following the research programme that the source itself identified as needed to confirm or falsify the scenario, and taking into account the updated constraints — the now-recommended initial 60^{60}Fe/56^{56}Fe of (1.01±0.27)×10−8(1.01±0.27)×10^{-8}, and 3D simulations of the same 11.8 solar-mass progenitor that find no fallback and give a ν-process 10^{10}Be yield reaching only 10^{10}Be/9^{9}Be of about 2×10−42×10^{-4} at most — the work on this single hypothesis comprises the following linked strands. (1) Explosion and injection physics: replace the older one-dimensional, parametrized-energy, piston-ejecta calculations with ad hoc fallback by self-consistent multi-dimensional (neutrino-radiation-hydrodynamic) simulations of low-mass progenitors and their explosions — quantifying fallback (late re-accretion of the innermost ejecta onto the newly formed neutron star) rather than assuming it — coupled to the evolution of the supernova remnant and its interaction with the surrounding interstellar medium and the proto-solar cloud, to determine whether the injection of short-lived radionuclides (SLRs) required for isotopic concordance — uniform or differential (tiered) between inner and outer ejecta, with dilution factor ff (the fraction of the radionuclide yield injected into the proto-solar cloud) and decay interval ΔΔ (the time between nucleosynthesis in the ejecta and incorporation into the first solar-system solids) — can actually be realized by that coupled evolution, with the injection efficiencies derived from it rather than parametrized to reproduce the isotopic data. (2) Radionuclide accounting: the scenario must simultaneously match the meteoritic early-solar-system abundances of the SLRs it attributes to the CCSN — notably 10^{10}Be, 41^{41}Ca and 107^{107}Pd, with 26^{26}Al, 36^{36}Cl, 53^{53}Mn, 60^{60}Fe, 135^{135}Cs, 182^{182}Hf and 205^{205}Pb not exceeded — under the now-recommended initial 60^{60}Fe/56^{56}Fe of (1.01±0.27)×10−8(1.01±0.27)×10^{-8}, keep stable-isotope shifts of Mg, Si, Ca and Fe below the meteoritic limit of ≈1≈1%, and settle whether the neutrino-process (ν-process) 10^{10}Be yield of such a supernova — in the combined budget sense of the source, i.e. possibly supplemented by remnant cosmic-ray production, to which the source adds a net cosmic-ray 10^{10}Be/9^{9}Be of about 10−410^{-4} — can account for the observed early-solar-system 10^{10}Be/9^{9}Be of (7.5±2.5)×10−4(7.5±2.5)×10^{-4}, given that current 3D-based yields fall short by a factor of several. (3) Nuclear input: clarify the nuclear physics of the effective stellar beta-decay rate of 181^{181}Hf, including the contribution of thermally populated low-lying excited states, which controls the predicted 182^{182}Hf yield and hence the s-process versus r-process attribution of the early-solar-system 182^{182}Hf. (4) Environmental context: comprehensive quantitative models of (a) enrichment of the swept-up interstellar medium in 26^{26}Al by winds of nearby massive stars that explode weakly or not at all, in an evolving giant molecular cloud, (b) cosmic-ray production and propagation in the evolving low-mass CCSN remnant as a possible supplementary source of 10^{10}Be by spallation, entering the same 10^{10}Be budget as strand (2), and (c) late-stage irradiation by solar energetic particles (SEPs, MeV-to-GeV ions emitted by the young Sun) of a volatile-rich (e.g., chlorine-enriched) reservoir as the origin of 36^{36}Cl without problematic coproduction of other SLRs, especially 10^{10}Be, 26^{26}Al and 53^{53}Mn, so that the CCSN contribution is not double-counted against these alternatives. (5) Meteoritic discriminant: assess the distinctive predicted ν-process Li-Be-B isotopic pattern — predominant production of 7^{7}Li and 11^{11}B, unlike cosmic-ray or SEP-irradiation patterns — against precise meteoritic Li, Be and B isotope data, distinguishing the CCSN signature from irradiation and molecular-cloud-inheritance explanations. These strands restate the programme of needed work that the source identified for a single hypothesis.

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